"Even slow-breeding man has doubled in twenty-five years, and at this rate, in less than a thousand years, there would literally not be standing-room for his progeny.

"Linnaeus has calculated that if an annual plant produced only two seeds- and there is no plant so unproductive as this- and their seedlings next year produced two, and so on, then in twenty years there should be a million plants.

"The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase. It will be safest to assume that it begins breeding when thirty years old, and goes on breeding till ninety years old, bringing forth six young in the interval, and surviving till one hundred years old.

"Hence we may confidently assert, that all plants and animals are tending to increase at a geometrical ratio -- that all would rapidly stock every station in which they could anyhow exist -- and that this geometrical tendency to increase must be checked by destruction at some period of life.

"Our familiarity with the larger domestic animals tends, I think, to mislead us. We see no great destruction falling on them, but we do not keep in mind that thousands are annually slaughtered for food, and that in a state of nature an equal number would have somehow to be disposed of.

"The only difference between organisms which annually produce eggs or seeds by the thousand -- and those which produce extremely few -- is, that the slow-breeders would require a few more years to people -- under favourable conditions -- a whole district, let it be ever so large.

"The condor lays a couple of eggs and the ostrich a score, and yet in the same country the condor may be the more numerous of the two.

"The Fulmar petrel lays but one egg, yet it is believed to be the most numerous bird in the world.

"One fly deposits hundreds of eggs, and another, like the hippobosca, a single one. But this difference does not determine how many individuals of the two species can be supported in a district.

"A large number of eggs is of some importance to those species which depend on a fluctuating amount of food, for it allows them rapidly to increase in number. The real importance of a large number of eggs or seeds is to make up for much destruction at some period of life.

"This period in the great majority of cases is an early one. If an animal can in any way protect its own eggs or young, a small number may be produced, and yet the average stock be fully kept up. But if many eggs or young are destroyed, many must be produced, or the species will become extinct.

"It would suffice to keep up the full number of a tree, which lived on an average for a thousand years, if a single seed were produced once in a thousand years -- supposing that this seed were never destroyed -- and could be ensured to germinate in a fitting place.

"That each lives by a struggle at some period of its life. That heavy destruction inevitably falls either on the young or old, during each generation or at recurrent intervals.

"Very frequently it is not the obtaining food, but the serving as prey to other animals, which determines the average numbers of a species. Thus, there seems to be little doubt that the stock of partridges, grouse, and hares on any large estate depends chiefly on the destruction of vermin.

"If not one head of game were shot during the next twenty years in England, and, at the same time, if no vermin were destroyed, there would, in all probability, be less game than at present, although hundreds of thousands of game animals are now annually shot.

"On the other hand, in some cases, as with the elephant, none are destroyed by beasts of prey. Even the tiger in India most rarely dares to attack a young elephant protected by its dam."

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"I estimated (chiefly from the greatly reduced numbers of nests in the spring) that the winter of 1854-5 destroyed four-fifths of the birds in my own grounds. This is a tremendous destruction, when we remember that ten per cent is an extraordinarily severe mortality from epidemics with man.

"The action of climate seems at first sight to be quite independent of the struggle for existence.

"But in so far as climate chiefly acts in reducing food, it brings on the most severe struggle between the individuals, whether of the same or of distinct species, which subsist on the same kind of food. Even when climate, for instance, extreme cold, acts directly, it will be the least vigorous individuals, or those which have got least food through the advancing winter, which will suffer most.

"When we travel from south to north, or from a damp region to a dry, we invariably see some species gradually getting rarer and rarer, and finally disappearing. The change of climate being conspicuous, we are tempted to attribute the whole effect to its direct action.

"This is a false view. We forget that each species, even where it most abounds, is constantly suffering enormous destruction at some period of its life, from enemies or from competitors for the same place and food.

"If these enemies or competitors be in the least degree favoured by any slight change of climate, they will increase in numbers. As each area is already fully stocked with inhabitants, the other species must decrease.

"When we travel southward and see a species decreasing in numbers, we may feel sure that the cause lies quite as much in other species being favoured, as in this one being hurt. So it is when we travel northward, but in a somewhat lesser degree. The number of species of all kinds, and therefore of competitors, decreases northwards. Hence in going northwards, or in ascending a mountain, we far oftener meet with stunted forms, due to the directly injurious action of climate, than we do in proceeding southwards or in descending a mountain.

"When we reach the arctic regions, or snowcapped summits, or absolute deserts, the struggle for life is almost exclusively with the elements.

"But even some of these so-called epidemics appear to be due to parasitic worms, which have from some cause, possibly in part through facility of diffusion amongst the crowded animals, been disproportionally favoured. Here comes in a sort of struggle between the parasite and its prey.

"On the other hand, in many cases, a large stock of individuals of the same species, relatively to the numbers of its enemies, is absolutely necessary for its preservation.

"Thus we can easily raise plenty of corn and rape-seed, &c., in our fields, because the seeds are in great excess compared with the number of birds which feed on them. Nor can the birds, though having a super-abundance of food at this one season, increase in number proportionally to the supply of seed, as their numbers are checked during the winter. Any one who has tried, knows how troublesome it is to get seed from a few wheat or other such plants in a garden. I have in this case lost every single seed.

"This view of the necessity of a large stock of the same species for its preservation, explains, I believe, some singular facts in nature, such as that of very rare plants being sometimes extremely abundant, in the few spots where they do exist -- and that of some social plants being social, that is abounding in individuals, even on the extreme verge of their range.

"In such cases, we may believe, that a plant could exist only where the conditions of its life were so favourable that many could exist together, and thus save the species from utter destruction.

"I will give only a single instance, which, though a simple one, interested me.

"In Staffordshire, on the estate of a relation, where I had ample means of investigation, there was a large and extremely barren heath, which had never been touched by the hand of man. Several hundred acres of exactly the same nature had been enclosed twenty-five years previously and planted with Scotch fir.

"The change in the native vegetation of the planted part of the heath was most remarkable, more than is generally seen in passing from one quite different soil to another. Not only the proportional numbers of the heath-plants were wholly changed, but twelve species of plants (not counting grasses and carices) flourished in the plantations, which could not be found on the heath.

"The effect on the insects must have been still greater, for six insectivorous birds were very common in the plantations, which were not to be seen on the heath. The heath was frequented by two or three distinct insectivorous birds.

"Here we see how potent has been the effect of the introduction of a single tree, nothing whatever else having been done, with the exception of the land having been enclosed, so that cattle could not enter.

"But how important an element enclosure is, I plainly saw near Farnham, in Surrey. Here there are extensive heaths, with a few clumps of old Scotch firs on the distant hilltops. Within the last ten years large spaces have been enclosed, and self-sown firs are now springing up in multitudes, so close together that all cannot live.

"When I ascertained that these young trees had not been sown or planted, I was so much surprised at their numbers that I went to several points of view, whence I could examine hundreds of acres of the unenclosed heath. Literally I could not see a single Scotch fir, except the old planted clumps.

"But on looking closely between the stems of the heath, I found a multitude of seedlings and little trees which had been perpetually browsed down by the cattle. In one square yard, at a point some hundred yards distant from one of the old clumps, I counted thirty-two little trees. One of them, with twenty-six rings of growth, had, during many years, tried to raise its head above the stems of the heath, and had failed.

"No wonder that, as soon as the land was enclosed, it became thickly clothed with vigorously growing young firs.

"Perhaps Paraguay offers the most curious instance of this. Here neither cattle nor horses nor dogs have ever run wild, though they swarm southward and northward in a feral state.

"Azara and Rengger have shown that this is caused by the greater number in Paraguay of a certain fly, which lays its eggs in the navels of these animals when first born. The increase of these flies, numerous as they are, must be habitually checked by some means, probably by other parasitic insects.

"Hence, if certain insectivorous birds were to decrease in Paraguay, the parasitic insects would probably increase. This would lessen the number of the navel-frequenting flies- then cattle and horses would become feral, and this would certainly greatly alter -- as indeed I have observed in parts of South America -- the vegetation. This again would largely affect the insects -- and this, as we have just seen in Staffordshire, the insectivorous birds -- and so onwards in ever-increasing circles of complexity.

"Not that under nature the relations will ever be as simple as this. Battle within battle must be continually recurring with varying success. Yet in the long run the forces are so nicely balanced, that the face of nature remains for long periods of time uniform, though assuredly the merest trifle would give the victory to one organic being over another.

"I shall hereafter have occasion to show that the exotic Lobelia fulgens is never visited in my garden by insects -- and consequently, from its peculiar structure, never sets a seed.

"Nearly all our orchidaceous plants absolutely require the visits of insects to remove their pollen-masses and thus to fertilise them. I find from experiments that bumble-bees are almost indispensable to the fertilisation of the heartsease (Viola tricolor), for other bees do not visit this flower.

"I have also found that the visits of bees are necessary for the fertilisation of some kinds of clover. For instance, 90 heads of Dutch clover (Trifolium repens) yielded 2,290 seeds, but 20 other heads protected from bees produced not one. Again, 100 heads of red clover (T. pratense) produced 2,700 seeds, but the same number of protected heads produced not a single seed. Bumble-bees alone visit red clover, as other bees cannot reach the nectar.

"It has been suggested that moths may fertilise the clovers. I doubt whether they could do so in the case of the red clover, from their weight not being sufficient to depress the wing petals.

"Hence we may infer as highly probable that, if the whole genus of bumble-bees became extinct or very rare in England, the heartsease and red clover would become very rare, or wholly disappear.

"The number of bumble-bees in any district depends in a great measure upon the number of field-mice, which destroy their combs and nests. Col. Newman, who has long attended to the habits of bumble-bees, believes that "more than two-thirds of them are thus destroyed all over England."

"But all will concur in determining the average number or even the existence of the species.

"In some cases it can be shown that widely-different checks act on the same species in different districts. When we look at the plants and bushes clothing an entangled bank, we are tempted to attribute their proportional numbers and kinds to what we call chance.

"But how false a view is this! Every one has heard that when an American forest is cut down a very different vegetation springs up. But it has been observed that ancient Indian ruins in the southern United States, which must formerly have been cleared of trees, now display the same beautiful diversity and proportion of kinds as in the surrounding virgin forest.

"What a struggle must have gone on during long centuries between the several kinds of trees each annually scattering its seeds by the thousand -- what war between insect and insect- between insects, snails, and other animals with birds and beasts of prey -- all striving to increase, all feeding on each other, or on the trees, their seeds and seedlings, or on the other plants which first clothed the ground and thus checked the growth of the trees!

"This is likewise sometimes the case with those which may be strictly said to struggle with each other for existence, as in the case of locusts and grass-feeding quadrupeds.

"But the struggle will almost invariably be most severe between the individuals of the same species, for they frequent the same districts, require the same food, and are exposed to the same dangers.

"In the case of varieties of the same species, the struggle will generally be almost equally severe, and we sometimes see the contest soon decided. For instance, if several varieties of wheat be sown together -- and the mixed seed be resown -- some of the varieties which best suit the soil or climate, or are naturally the most fertile, will beat the others and so yield more seed -- and will consequently in a few years supplant the other varieties.

"To keep up a mixed stock of even such extremely close varieties as the variously-coloured sweet peas, they must be each year harvested separately -- and the seed then mixed in due proportion -- otherwise the weaker kinds will steadily decrease in number and disappear.

"So again with the varieties of sheep. It has been asserted that certain mountain-varieties will starve out other mountain-varieties, so that they cannot be kept together.

"The same result has followed from keeping together different varieties of the medicinal leech.

"We see this in the recent extension over parts of the United States of one species of swallow having caused the decrease of another species.

"The recent increase of the missel-thrush in parts of Scotland has caused the decrease of the song-thrush.

"How frequently we hear of one species of rat taking the place of another species under the most different climates!

"In Russia the small Asiatic cockroach has everywhere driven before it its great congener.

"In Australia the imported hive-bee is rapidly exterminating the small, stingless native bee.

"One species of charlock has been known to supplant another species -- and so in other cases.

"This is obvious in the structure of the teeth and talons of the tiger -- and in that of the legs and claws of the parasite which clings to the hair on the tiger's body.

"But in the beautifully plumed seed of the dandelion, and in the flattened and fringed legs of the water-beetle, the relation seems at first confined to the elements of air and water.

"Yet the advantage of plumed seeds no doubt stands in the closest relation to the land being already thickly clothed with other plants -- so that the seeds may be widely distributed and fall on unoccupied ground.

"But from the strong growth of young plants produced from such seeds -- as peas and beans, when sown in the midst of long grass -- it may be suspected that the chief use of the nutriment in the seed is to favour the growth of the seedlings, whilst struggling with other plants growing vigorously all around.

"Look at a plant in the midst of its range. Why does it not double or quadruple its numbers?

"We know that it can perfectly well withstand a little more heat or cold, dampness or dryness. Elsewhere it ranges into slightly hotter or colder, damper or drier districts.

"In this case we can clearly see that if we wish in imagination to give the plant the power of increasing in number, we should have to give it some advantage over its competitors, or over the animals which prey on it.

"On the confines of its geographical range, a change of constitution with respect to climate would clearly be an advantage to our plant. But we have reason to believe that only a few plants or animals range so far, that they are destroyed exclusively by the rigour of the climate.

"If its average numbers are to increase in its new home, we should have to modify it in a different way to what we should have had to do in its native country. We should have to give it some advantage over a different set of competitors or enemies.

"It is good thus to try in imagination to give to any one species an advantage over another. Probably in no single instance should we know what to do.

"This ought to convince us of our ignorance on the mutual relations of all organic beings -- a conviction as necessary as it is difficult to acquire.

"All that we can do, is to keep steadily in mind that each organic being is striving to increase in a geometrical ratio. Each at some period of its life, during some season of the year, during each generation or at intervals, has to struggle for life and to suffer great destruction.

"Can the principle of selection, which we have seen is so potent in the hands of man, apply under nature? I think we shall see that it can act most efficiently.

"Let the endless number of slight variations and individual differences occurring in our domestic productions -- and, in a lesser degree, in those under nature -- be borne in mind -- as well as the strength of the hereditary tendency.

"Under domestication, it may be truly said that the whole organisation becomes in some degree plastic.

"But the variability, which we almost universally meet with in our domestic productions, is not directly produced, as Hooker and Asa Gray have well remarked, by man. He can neither originate varieties, nor prevent their occurrence. He can preserve and accumulate such as do occur.

"Unintentionally he exposes organic beings to new and changing conditions of life, and variability ensues. But similar changes of conditions might and do occur under nature.

"Let it also be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life -- and consequently what infinitely varied diversities of structure might be of use to each being under changing conditions of life.

"Can it, then, be thought improbable -- seeing that variations useful to man have undoubtedly occurred -- that other variations useful in some way to each being in the great and complex battle of life, should occur in the course of many successive generations?

"If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind?

"On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed.

"This preservation of favourable individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection, or the Survival of the Fittest.

"No one objects to agriculturists speaking of the potent effects of man's selection. In this case the individual differences given by nature, which man for some object selects, must of necessity first occur.

"Others have objected that the term selection implies conscious choice in the animals which become modified. It has even been urged that, as plants have no volition, natural selection is not applicable to them!

"In the literal sense of the word, no doubt, natural selection is a false term. But who ever objected to chemists speaking of the elective affinities of the various elements? Yet an acid cannot strictly be said to elect the base with which it in preference combines.

"It has been said that I speak of natural selection as an active power or Deity. But who objects to an author speaking of the attraction of gravity as ruling the movements of the planets?

"Every one knows what is meant and is implied by such metaphorical expressions -- and they are almost necessary for brevity.

"So again it is difficult to avoid personifying the word Nature. I mean by Nature, only the aggregate action and product of many natural laws -- and by laws the sequence of events as ascertained by us.

"The proportional numbers of its inhabitants will almost immediately undergo a change, and some species will probably become extinct.

"We may conclude, from what we have seen of the intimate and complex manner in which the inhabitants of each country are bound together, that any change in the numerical proportions of the inhabitants, independently of the change of climate itself, would seriously affect the others.

"If the country were open on its borders, new forms would certainly immigrate, and this would likewise seriously disturb the relations of some of the former inhabitants.

"Let it be remembered how powerful the influence of a single introduced tree or mammal has been shown to be.

"In the case of an island, or of a country partly surrounded by barriers, into which new and better adapted forms could not freely enter, we should then have places in the economy of nature which would assuredly be better filled up, if some of the original inhabitants were in some manner modified.

"In the foregoing cases the conditions have changed, and this would manifestly be favourable to natural selection, by affording a better chance of the occurrence of profitable variations.

"Unless such occur, natural selection can do nothing. Under the term of "variations," it must never be forgotten that mere individual differences are included.

"As man can produce a great result with his domestic animals and plants by adding up in any given direction individual differences, so could natural selection, but far more easily from having incomparably longer time for action.

"Nor do I believe that any great physical change, as of climate, or any unusual degree of isolation to check immigration, is necessary in order that new and unoccupied places should be left, for natural selection to fill up by improving some of the varying inhabitants. As all the inhabitants of each country are struggling together with nicely balanced forces, extremely slight modifications in the structure or habits of one species would often give it an advantage over others.

"Still further modifications of the same kind would often still further increase the advantage, as long as the species continued under the same conditions of life and profited by similar means of subsistence and defence.

"Man can act only on external and visible characters: Nature, if I may be allowed to personify the natural preservation or survival of the fittest, cares nothing for appearances, except in so far as they are useful to any being.

"She can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life.

"Man selects only for his own good -- nature only for that of the being which she tends. Every selected character is fully exercised by her, as is implied by the fact of their selection.

"Man keeps the natives of many climates in the same country.

"He seldom exercises each selected character in some peculiar and fitting manner. He feeds a long and a short beaked pigeon on the same food. He does not exercise a long-backed or long-legged quadruped in any peculiar manner. He exposes sheep with long and short wool to the same climate.

"He does not allow the most vigorous males to struggle for the females.

"He does not rigidly destroy all inferior animals, but protects during each varying season -- as far as lies in his power -- all his productions.

"He often begins his selection by some half-monstrous form -- or at least by some modification prominent enough to catch the eye or to be plainly useful to him.

"Under nature, the slightest differences of structure or constitution may well turn the nicely balanced scale in the struggle for life, and so be preserved.

"How fleeting are the wishes and efforts of man! how short his time! and consequently how poor will be his results, compared with those accumulated by Nature during whole geological periods!

"We see nothing of these slow changes in progress, until the hand of time has marked the lapse of ages, and then so imperfect is our view into long-past geological ages, that we see only that the forms of life are now different from what they formerly were.

"In order that any great amount of modification should be effected in a species, a variety when once formed must again -- perhaps after a long interval of time -- vary or present individual differences of the same favourable nature as before.

"These must be again preserved, and so onwards step by step. Seeing that individual differences of the same kind perpetually recur, this can hardly be considered as an unwarrantable assumption.

"But whether it is true, we can judge only by seeing how far the hypothesis accords with and explains the general phenomena of nature.

"When we see leaf-eating insects green -- and bark-feeders mottled-grey -- the alpine ptarmigan white in winter -- the red grouse the colour of heather -- we must believe that these tints are of service to these birds and insects in preserving them from danger.

"Grouse, if not destroyed at some period of their lives, would increase in countless numbers. They are known to suffer largely from birds of prey. Hawks are guided by eyesight to their prey- so much so, that on parts of the Continent persons are warned not to keep white pigeons, as being the most liable to destruction.

"Hence natural selection might be effective in giving the proper colour to each kind of grouse, and in keeping that colour, when once acquired, true and constant.

"Nor ought we to think that the occasional destruction of an animal of any particular colour would produce little effect. We should remember how essential it is in a flock of white sheep to destroy a lamb with the faintest trace of black.

"We have seen how the colour of the hogs, which feed on the "paint-root" in Virginia, determines whether they shall live or die.

"In plants, the down on the fruit and the colour of the flesh are considered by botanists as characters of the most trifling importance. Yet we hear from an excellent horticulturist, Downing, that in the United States, smooth-skinned fruits suffer far more from a beetle, a Curculio, than those with down -- that purple plums suffer far more from a certain disease than yellow plums -- whereas another disease attacks yellow-fleshed peaches far more than those with other coloured flesh.

"If it profit a plant to have its seeds more and more widely disseminated by the wind, I can see no greater difficulty in this being effected through natural selection, than in the cotton-planter increasing and improving by selection the down in the pods on his cotton-trees.

"Natural selection may modify and adapt the larva of an insect to a score of contingencies, wholly different from those which concern the mature insect. These modifications may affect, through correlation, the structure of the adult.

"In social animals it will adapt the structure of each individual for the benefit of the whole community, if the community profits by the selected change.

"What natural selection cannot do, is to modify the structure of one species, without giving it any advantage, for the good Of another species. Though statements to this effect may be found in works of natural history, I cannot find one case which will bear investigation.

"A structure used only once in an animal's life, if of high importance to it, might be modified to any extent by natural selection. For instance, the great jaws possessed by certain insects, used exclusively for opening the cocoon- or the hard tip to the beak of unhatched birds, used for breaking the egg.

"It has been asserted, that of the best short-beaked tumbler-pigeons a greater number perish in the egg than are able to get out of it so that fanciers assist in the act of hatching.

"For instance a vast number of eggs or seeds are annually devoured, and these could be modified through natural selection only if they varied in some manner which protected them from their enemies. Yet many of these eggs or seeds would perhaps, if not destroyed, have yielded individuals better adapted to their conditions of life than any of these which happened to survive.

"So again a vast number of mature animals and plants, whether or not they be the best adapted to their conditions, must be annually destroyed by accidental causes -- which would not be in the least degree mitigated by certain changes of structure or constitution which would in other ways be beneficial to the species.

"But let the destruction of the adults be ever so heavy -- if the number which can exist in any district be not wholly kept down by such causes,- or again let the destruction of eggs or seeds be so great that only a hundredth or a thousandth part are developed -- yet of those which do survive, the best adapted individuals, supposing that there is any variability in favourable direction, will tend to propagate their kind in larger numbers than the less well adapted.

"Thus it is rendered possible for the two sexes to be modified through natural selection in relation to different habits of life, as is sometimes the case -- or for one sex to be modified in relation to the other sex, as commonly occurs.

"This leads me to say a few words on what I have called Sexual Selection. This form of selection depends, not on a struggle for existence in relation to other organic beings or to external conditions, but on a struggle between the individuals of one sex -- generally the males -- for the possession of the other sex. The result is not death to the unsuccessful competitor, but few or no offspring.

"Sexual selection is, therefore, less rigorous than natural selection.

"Generally, the most vigorous males, those which are best fitted for their places in nature, will leave most progeny. But in many cases, victory depends not so much on general vigor, as on having special weapons, confined to the male sex.

"A hornless stag or spurless cock would have a poor chance of leaving numerous offspring. Sexual selection, by always allowing the victor to breed, might surely give indomitable courage -- length to the spur -- and strength to the wing to strike in the spurred leg -- in nearly the same manner as does the brutal cockfighter by the careful selection of his best cocks.

"How low in the scale of nature the law of battle descends, I know not. Male alligators have been described as fighting, bellowing, and whirling round, like Indians in a war-dance, for the possession of the females. Male salmons have been observed fighting all day long. Male stagbeetles sometimes bear wounds from the huge mandibles of other males. The males of certain hymenopterous insects have been frequently seen by that inimitable observer M. Fabre, fighting for a particular female who sits by, an apparently unconcerned beholder of the struggle, and then retires with the conqueror.

"The war is, perhaps, severest between the males of polygamous animals, and these seem oftenest provided with special weapons.

"The males of carnivorous animals are already well armed -- though to them and to others, special means of defence may be given through means of sexual selection -- as the mane of the lion, and the hooked jaw to the male salmon.

"Yet, I would not wish to attribute all sexual differences to this agency. We see in our domestic animals peculiarities arising and becoming attached to the male sex, which apparently have not been augmented through selection by man.

"Let us take the case of a wolf, which preys on various animals, securing some by craft, some by strength, and some by fleetness.

"Let us suppose that the fleetest prey, a deer for instance, had from any change in the country increased in numbers -- or that other prey had decreased in numbers, during that season of the year when the wolf was hardest pressed for food.

"Under such circumstances the swiftest and slimmest wolves would have the best chance of surviving and so be preserved or selected -- provided always that they retained strength to master their prey at this or some other period of the year, when they were compelled to prey on other animals.

"I can see no more reason to doubt that this would be the result, than that man should be able to improve the fleetness of his greyhounds by careful and methodical selection, or by that kind of unconscious selection which follows from each man trying to keep the best dogs without any thought of modifying the breed.

"In former editions of this work I sometimes spoke as if this latter alternative had frequently occurred.

"I saw the great importance of individual differences, and this led me fully to discuss the results of unconscious selection by man, which depends on the preservation of all the more or less valuable individuals, and on the destruction of the worst.

"I saw, also, that the preservation in a state of nature of any occasional deviation of structure, such as a monstrosity, would be a rare event -- and that, if at first preserved, it would generally be lost by subsequent intercrossing with ordinary individuals.

"Nevertheless, until reading an able and valuable article in the North British Review (1867), I did not appreciate how rarely single variations, whether slight or strongly-marked, could be. perpetuated. The author takes the case of a pair of animals, producing during their lifetime two hundred offspring, of which, from various causes of destruction, only two on an average survive to procreate their kind.

"This is rather an extreme estimate for most of the higher animals, but by no means so for many of the lower organisms.

"He then shows that if a single individual were born, which varied in some manner -- giving it twice as good a chance of life as that of the other individuals -- yet the chances would be strongly against its survival.

"Supposing it to survive and to breed, and that half its young inherited the favourable variation -- still, as the reviewer goes on to show, the young would have only a slightly better chance of surviving and breeding. This chance would go on decreasing in the succeeding generations.

"The justice of these remarks cannot, I think, be disputed. If, for instance, a bird of some kind could procure its food more easily by having its beak curved -- and if one were born with its beak strongly curved, and which consequently flourished -- nevertheless there would be a very poor chance of this one individual perpetuating its kind to the exclusion of the common form.

"In such cases, if the varying individual did not actually transmit to its offspring its newly-acquired character, it would undoubtedly transmit to them -- as long as the existing conditions remained the same -- a still stronger tendency to vary in the same manner.

"There can also be little doubt that the tendency to vary in the same manner has often been so strong that all the individuals of the same species have been similarly modified without the aid of any form of selection.

"Only a third, fifth, or tenth part of the individuals may have been thus affected, of which fact several instances could be given.

"Thus Graba estimates that about one-fifth of the guillemots in the Faroe Islands consist of a variety so well marked, that it was formerly ranked as a distinct species under the name of Uria lacrymans.

"But it may be here remarked that most animals and plants keep to their proper homes, and do not needlessly wander about. We see this even with migratory birds, which almost always return to the same spot.

"Consequently each newly-formed variety would generally be at first local, as seems to be the common rule with varieties in a state of nature. So that similarly modified individuals would soon exist in a small body together, and would often breed together.

"Certain plants excrete sweet juice, apparently for the sake of eliminating something injurious from the sap. This is effected, for instance, by glands at the base of the stipules in some Leguminosae and at the backs of the leaves of the common laurel.

"This juice, though small in quantity, is greedily sought by insects. But their visits do not in any way benefit the plant.

"Now, let us suppose that the juice or nectar was excreted from the inside of the flowers of a certain number of plants of any species. Insects in seeking the nectar would get dusted with pollen, and would often transport it from one flower to another.

"The flowers of two distinct individuals of the same species would thus get crossed; and the act of crossing, as can be fully proved, gives rise to vigorous seedlings which consequently would have the best chance of flourishing and surviving

"The plants which produced flowers with the largest glands or nectaries, excreting most nectar, would oftenest be visited by insects, and would oftenest be crossed -- and so in the long run would gain the upper hand and form a local variety.

"The flowers, also, which had their stamens and pistils placed, in relation to the size and habits of the particular insects which visited them -- so as to favour in any degree the transportal of the pollen -- would likewise be favoured.

"We might have taken the case of insects visiting flowers for the sake of collecting pollen instead of nectar, As pollen is formed for the sole purpose of fertilisation, its destruction appears to be a simple loss to the plant. Yet if a little pollen were carried, at first occasionally and then habitually, by the pollen-devouring insects from flower to flower, and a cross thus effected -- although nine-tenths of the pollen were destroyed it might still be a great gain to the plant to be thus robbed.

"That they do this effectually, I could easily show by many striking facts.

"I will give only one, as likewise illustrating one step in the separation of the sexes of plants. Some holly-trees bear only male flowers, which have four stamens producing a rather small quantity of pollen, and a rudimentary pistil. Other holly-trees bear only female flowers. These have a full-sized pistil, and four stamens with shrivelled anthers, in which not a grain of pollen can be detected.

"Having found a female tree exactly sixty yards from a male tree, I put the stigmas of twenty flowers, taken from different branches, under the microscope. On all, without exception, there were a few pollen grains, and on some a profusion.

"As the wind had set for several days from the female to the male tree, the pollen could not thus have been carried. The weather had been cold and boisterous, and therefore not favourable to bees. Nevertheless every female flower which I examined had been effectually fertilised by the bees, which had flown from tree to tree in search of nectar.

"But to return to our imaginary case -- as soon as the plant had been rendered so highly attractive to insects that pollen was regularly carried from flower to flower, another process might commence.

"No naturalist doubts the advantage of what has been called the "physiological division of labour". Hence we may believe that it would be advantageous to a plant to produce stamens alone in one flower or on one whole plant, and pistils alone in another flower or on another plant.

"In plants under culture and placed under new conditions of life, sometimes the male organs and sometimes the female organs become more or less impotent. Now if we suppose this to occur in ever so slight a degree under nature -- then, as pollen is already carried regularly from flower to flower -- and as a more complete separation of the sexes of our plant would be advantageous on the principle of the division of labour -- individuals with this tendency more and more increased, would be continually favoured or selected, until at last a complete separation of the sexes might be effected.

"But we now seldom hear the agencies which we see still at work, spoken of as trifling or insignificant, when used in explaining the excavation of the deepest valleys or the formation of long lines of inland cliffs.

"Natural selection acts only by the preservation and accumulation of small inherited modifications, each profitable to the preserved being.

"In the case of animals and plants with separated sexes, it is of course obvious that two individuals must always (with the exception of the curious and not well-understood cases of parthenogenesis) unite for each birth.

"But in the case of hermaphrodites this is far from obvious. Nevertheless there is reason to believe that with all hermaphrodites two individuals, either occasionally or habitually, concur for the reproduction of their kind.

"This view was long ago doubtfully suggested by Sprengel, Knight and Kolreuter. We shall presently see its importance. But I must here treat the subject with extreme brevity, though I have the materials prepared for an ample discussion.

"All vertebrate animals, all insects, and some other large groups of animals, pair for each birth. Modern research has much diminished the number of supposed hermaphrodites, and of real hermaphrodites a large number pair -- that is, two individuals regularly unite for reproduction, which is all that concerns us.

"But still there are many hermaphrodite animals which certainly do not habitually pair, and a vast majority of plants are hermaphrodites.

"On the other hand, that close interbreeding diminishes vigour and fertility -- that these facts alone incline me to believe that it is a general law of nature that no organic being fertilises itself for a perpetuity of generations.

"Every hybridizer knows how unfavourable exposure to wet is to the fertilisation of a flower, yet what a multitude of flowers have their anthers and stigmas fully exposed to the weather!

"If an occasional cross be indispensable -- notwithstanding that the plant's own anthers and pistil stand so near each other as almost to insure self-fertilisation -- the fullest freedom for the entrance of pollen from another individual will explain the above state of exposure of the organs.

"Many flowers, on the other hand, have their organs of fructification closely enclosed, as in the great papilionaceous or pea-family. These almost invariably present beautiful and curious adaptations in relation to the visits of insects.

"So necessary are the visits of bees to many papilionaceous flowers, that their fertility is greatly diminished if these visits be prevented. It is scarcely possible for insects to fly from flower and flower, and not to carry pollen from one to the other, to the great good of the plant.

"Insects act like a camel-hair pencil, and it is sufficient to ensure fertilisation, just to touch with the same brush the anthers of one flower and then the stigma of another. But it must not be supposed that bees would thus produce a multitude of hybrids between distinct species.

"No doubt it is useful for this end. But the agency of insects is often required to cause the stamens to spring forward, as Kolreuter has shown to be the case with the barberry.

"In this very genus, which seems to have a special contrivance for self-fertilisation, it is well known that -- if closely allied forms or varieties are planted near each other -- it is hardly possible to raise pure seedlings, so largely do they naturally cross.

"In numerous other cases -- far from self-fertilisation being favoured -- there are special contrivances which effectually prevent the stigma receiving pollen from its own flower -- as I could show from the works of Sprengel and others, as well as from my own observations.

"For instance, in Lobelia fulgens, there is a really beautiful and elaborate contrivance by which all the infinitely numerous pollen-granules are swept out of the conjoined anthers of each flower, before the stigma of that individual flower is ready to receive them.

"As this flower is never visited, at least in my garden, by insects, it never sets a seed, though by placing pollen from one flower on the stigma of another, I raise plenty of seedlings.

"Another species of Lobelia which is visited by bees, seeds freely in my garden. In very many other cases, though, there is no special mechanical contrivance to prevent the stigma receiving pollen from the same flower.

"Yet, as Sprengel, and more recently Hildebrand, and others, have shown, and as I can confirm, either the anthers burst before the stigma is ready for fertilisation, or the stigma is ready before the pollen of that flower is ready.

"These so-named dichogamous plants have in fact separated sexes, and must habitually be crossed.

"So it is with the reciprocally dimorphic and trimorphic plants previously alluded to.

"How strange are these facts! How strange that the pollen and stigmatic surface of the same flower, though placed so close together, as if for the very purpose of self-fertilisation, should be in so many cases mutually useless to each other!

"For instance, I raised 233 seedling cabbages from some plants of different varieties growing near each other, and of these only 78 were true to their kind, and some even of these were not perfectly true.

"Yet the pistil of each cabbage-flower is surrounded not only by its own six stamens but by those of the many other flowers on the same plant. And the pollen of each flower readily gets on its own stigma without insect agency. I have found that plants carefully protected from insects produce the full number of pods.

"How, then, comes it that such a vast number of the seedlings are mongrelized? It must arise from the pollen of a distinct variety having a prepotent effect over the flower's own pollen -- and that this is part of the general law of good being derived from the intercrossing of distinct individuals of the same species.

"When distinct species are crossed the case is reversed, for a plant's own pollen is almost always prepotent over foreign pollen.

"I believe this objection to be valid, but that nature has largely provided against it by giving to trees a strong tendency to bear flowers with separated sexes.

"When the sexes are separated, although the male and female flowers may be produced on the same tree, pollen must be regularly carried from flower to flower. This will give a better chance of pollen being occasionally carried from tree to tree.

"That trees belonging to all Orders have their sexes more often separated than other plants, I find to be the case in this country.

"At my request Dr. Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those of the United States. The result was as I anticipated.

"On the other hand, Dr. Hooker informs me that the rule does not hold good in Australia but if most of the Australian trees are dichogamous, the same result would follow as if they bore flowers with separated sexes.

"As yet I have not found a single terrestrial animal which can fertilise itself.

"This remarkable fact -- which offers so strong a contrast with terrestrial plants -- is intelligible on the view of an occasional cross being indispensable. For owing to the nature of the fertilising element there are no means -- analogous to the action of insects and of the wind with plants -- by which an occasional cross could be effected with terrestrial animals without the concurrence of two individuals.

"Of aquatic animals, there are many self-fertilizing hermaphrodites. But here the currents of water offer an obvious means for an occasional cross.

"As in the case of flowers, I have as yet failed -- after consultation with one of the highest authorities, namely, Professor Huxley -- to discover a single hermaphrodite animal with the organs of reproduction so perfectly enclosed that access from without, and the occasional influence of a distinct individual, can be shown to be physically impossible.

"A great amount of variability -- under which term individual differences are always included -- will evidently be favourable. A large number of individuals -- by giving a better chance within any given period for the appearance of profitable variations -- will compensate for a lesser amount of variability in each individual, and is, I believe, a highly important element of success.

"Though Nature grants long periods of time for the work of natural selection, she does not grant an indefinite period. As all organic beings are striving to seize on each place in the economy of nature, if any one species does not become modified and improved in a corresponding degree with its competitors, it will be exterminated.

"Unless favourable variations be inherited by some at least of the offspring, nothing can be effected by natural selection.

"But when many men, without intending to alter the breed, have a nearly common standard of perfection -- and all try to procure and breed from the best animals -- improvement surely but slowly follows from this unconscious process of selection -- notwithstanding that there is no separation of selected individuals.

"Thus it will be under nature. Within a confined area -- with some place in the natural polity not perfectly occupied -- all the individuals varying in the right direction, though in different degrees, will tend to be preserved.

"But if the area be large, its several districts will almost certainly present different conditions of life. Then, if the same species undergoes modification in different districts, the newly-formed varieties will intercross on the confines of each.

"But we shall see in the sixth chapter that intermediate varieties, inhabiting intermediate districts, will in the long run generally be supplanted by one of the adjoining varieties. Intercrossing will chiefly affect those animals which unite for each birth and wander much, and which do not breed at a very quick rate.

"Hence with animals of this nature -- for instance, birds -- varieties will generally be confined to separated countries. This I find to be the case.

"With hermaphrodite organisms which cross only occasionally -- and likewise with animals which unite for each birth, but which wander little and can increase at a rapid rate -- a new and improved variety might be quickly formed on any one spot, and might there maintain itself in a body and afterwards spread, so that the individuals of the new variety would chiefly cross together.

"It will obviously thus act far more efficiently with those animals which unite for each birth. But, as already stated, we have reason to believe that occasional intercrosses take place with all animals and plants.

"Even if these take place only at long intervals of time -- the young thus produced will gain so much in vigour and fertility over the offspring from long-continued self-fertilisation, that they will have a better chance of surviving and propagating their kind. Thus in the long run the influence of crosses, even at rare intervals, will be great.

"With respect to organic beings extremely low in the scale -- which do not propagate sexually, nor conjugate, and which cannot possibly intercross -- uniformity of character can be retained by them under the same conditions of life, only through the principle of inheritance, and through natural selection which will destroy any individuals departing from the proper type.

"In a confined or isolated area, if not very large, the organic and inorganic conditions of life will generally be almost uniform. Natural selection will tend to modify all the varying individuals of the same species in the same manner.

"Intercrossing with the inhabitants of the surrounding districts will, also, be thus prevented.

"Moritz Wagner has lately published an interesting essay on this subject, and has shown that the service rendered by isolation in preventing crosses between newly-formed varieties is probably greater even than I supposed. But from reasons already assigned I can by no means agree with this naturalist, that migration and isolation are necessary elements for the formation of new species.

"The importance of isolation is likewise great in preventing, after any physical change in the conditions, such as of climate, elevation of the land, &c., the immigration of better adapted organisms.

"Thus new places in the natural economy of the district will be left open to be filled up by the modification of the old inhabitants.

"Lastly, isolation will give time for a new variety to be improved at a slow rate. This may sometimes be of much importance.

"I state this because it has been erroneously asserted that the element of time has been assumed by me to play an all-important part in modifying species, as if all the forms of life were necessarily undergoing change through some innate law.

"Lapse of time is only so far important -- and its importance in this respect is great -- that it gives a better chance of beneficial variations arising and of their being selected, accumulated, and fixed.

"Hence an oceanic island at first sight seems to have been highly favourable for the production of new species.

"Throughout a great and open area, not only will there be a better chance of favourable variations, arising from the large number of individuals of the same species there supported -- but the conditions of life are much more complex from the large number of already existing species.

"If some of these many species become modified and improved, others will have to be improved in a corresponding degree, or they will be exterminated.

"Each new form, also, as soon as it has been much improved, will be able to spread over the open and continuous area, and will thus come into competition with many other forms.

"Moreover, great areas, though now continuous, will often, owing to former oscillations of level, have existed in a broken condition.

"The good effects of isolation will generally, to a certain extent, have concurred.

"Finally, I conclude that, although small isolated areas have been in some respects highly favourable for the production of new species, yet that the course of modification will generally have been more rapid on large areas. What is more important, that the new forms produced on large areas -- which already have been victorious over many competitors -- will be those that will spread most widely, and will give rise to the greatest number of new varieties and species.

"For instance, the fact of the productions of the smaller continent of Australia now yielding before those of the larger Europaeo-Asiatic area.

"Thus, also, it is that continental productions have everywhere become so largely naturalised on islands.

"On a small island, the race for life will have been less severe, and there will have been less modification and less extermination. Hence, we can understand how it is that the flora of Madeira, according to Oswald Heer, resembles to a certain extent the extinct tertiary flora of Europe.

"All fresh-water basins, taken together, make a small area compared with that of the sea or of the land. Consequently, the competition between fresh-water productions will have been less severe than elsewhere. New forms will have been then more slowly produced, and old forms more slowly exterminated.

"And it is in fresh-water basins that we find seven genera of Ganoid fishes, remnants of a once preponderant order. In fresh water we find some of the most anomalous forms now known in the world as the Ornithorhynchus and Lepidosiren which, like fossils, connect to a certain extent orders at present widely sundered in the natural scale.

"I conclude that for terrestrial productions a large continental area -- which has undergone many oscillations of level -- will have been the most favourable for the production of many new forms of life -- fitted to endure for a long time and to spread widely.

"Whilst the area existed as a continent, the inhabitants will have been numerous in individuals and kinds, and will have been subjected to severe competition.

"When converted by subsidence into large separate islands, there will still have existed many individuals of the same species on each island. Intercrossing on the confines of the range of each new species will have been checked. After physical changes of any kind, immigration will have been prevented, so that new places in the polity of each island will have had to be filled up by the modification of the old inhabitants.

"Time will have been allowed for the varieties in each to become well modified and perfected.

"When, by renewed elevation, the islands were reconverted into a continental area, there will again have been very severe competition. The most favoured or improved varieties will have been enabled to spread. There will have been much extinction of the less improved forms.

"It can act only when there are places in the natural polity of a district which can be better occupied by the modification of some of its existing inhabitants. The occurrence of such places will often depend on physical changes -- which generally take place very slowly -- and on the immigration of better adapted forms being prevented.

"As some few of the old inhabitants become modified, the mutual relations of others will often be disturbed. This will create new places, ready to be filled up by better adapted forms, but all this will take place very slowly.

"Although the individuals of the same species differ in some slight degree from each other, it would often be long before differences of the right nature in various parts of the organisation might occur. The result would often be greatly retarded by free intercrossing.

"Natural selection acts solely through the preservation of variations in some way advantageous, which consequently endure.

"Owing to the high geometrical rate of increase of all organic beings, each area is already fully stocked with inhabitants. It follows from this, that as the favoured forms increase in number, so, generally, will the less favoured decrease and become rare.

"Rarity, as geology tells us, is the precursor to extinction. We can see that any form which is represented by few individuals will run a good chance of utter extinction -- during great fluctuations in the nature of the seasons -- or from a temporary increase in the number of its enemies.

"The forms which stand in closest competition with those undergoing modification and improvement will naturally suffer most.

"And we have seen in the chapter on the Struggle for Existence that it is the most closely-allied forms -- varieties of the same species, and species of the same genus or of related genera -- which, from having nearly the same structure, constitution, and habits, generally come into the severest competition with each other.

"Consequently, each new variety or species, during the progress of its formation, will generally press hardest on its nearest kindred -- and tend to exterminate them.

"We see the same process of extermination amongst our domesticated productions, through the selection of improved forms by man. Many curious instances could be given showing how quickly new breeds of cattle, sheep, and other animals, and varieties of flowers, take the place of older and inferior kinds.

"In the first place, varieties, even strongly-marked ones, though having somewhat of the character of species -- as is shown by the hopeless doubts in many cases how to rank them -- yet certainly differ far less from each other than do good and distinct species.

"Nevertheless, according to my view, varieties are species in the process of formation -- or are, as I have called them, incipient species.

"How, then, does the lesser difference between varieties become augmented into the greater difference between species? That this does habitually happen, we must infer from most of the innumerable species throughout nature presenting well-marked differences.

"Whereas varieties, the supposed prototypes and parents of future well-marked species, present slight and ill-defined differences. Mere chance, as we may call it, might cause one variety to differ in some character from its parents -- and the offspring of this variety again to differ from its parent in the very same character and in a greater degree.

"We shall here find something analogous. It will be admitted that the production of races so different as short-horn and Hereford cattle -- race and cart horses -- the several breeds of pigeons, &c. -- could never have been effected by the mere chance accumulation of similar variations during many successive generations.

"In practice, a fancier is, for instance, struck by a pigeon having a slightly shorter beak. Another fancier is struck by a pigeon having a rather longer beak.

"On the acknowledged principle that "fanciers do not and will not admire a medium standard, but like extremes," they both go on (as has actually occurred with the sub-breeds of the tumbler-pigeon) choosing and breeding from birds with longer and longer beaks, or with shorter and shorter beaks.

"Again, we may suppose that at an early period of history, the men of one nation or district required swifter horses, whilst those of another required stronger and bulkier horses. The early differences would be very slight. But, in the course of time from the continued selection of swifter horses in the one case, and of stronger ones in the other, the differences would become greater, and would be noted as forming two sub-breeds.

"Ultimately, after the lapse of centuries, these sub-breeds would become converted into two well-established and distinct breeds. As the differences became greater, the inferior animals with intermediate characters, being neither swift nor very strong, would not have been used for, breeding, and will thus have tended to disappear.

"Take the case of a carnivorous quadruped, of which the number that can be supported in any country has long ago arrived at its full average. If its natural power of increase be allowed to act, it can succeed in increasing -- the country not undergoing any change in conditions -- only by its varying descendants seizing on places at present occupied by other animals -- some of them, for instance, being enabled to feed on new kinds of prey, either dead or alive; some inhabiting new stations, climbing trees, frequenting water, and some perhaps becoming less carnivorous.

"The more diversified in habits and structure the descendants of our carnivorous animals become, the more places they will be enabled to occupy.

"What applies to one animal will apply throughout all time to all animals- that is, if they vary- for otherwise natural selection can effect nothing.

"So it will be with plants. It has been experimentally proved, that if a plot of ground be sown with one species of grass, and a similar plot be sown with several distinct genera of grasses, a greater number of plants and a greater weight of dry herbage can be raised in the latter than in the former case. The same has been found to hold good when one variety and several mixed varieties of wheat have been sown on equal spaces of ground.

"Hence, if any one species of grass were to go on varying, and the varieties were continually selected which differed from each other in the same manner -- though in a very slight degree, as do the distinct species and genera of grasses -- a greater number of individual plants of this species, including its modified descendants, would succeed in living on the same piece of ground.

"And we know that each species and each variety of grass is annually sowing almost countless seeds; and is thus striving, as it may be said, to the utmost to increase in number.

"Consequently, in the course of many thousand generations, the most distinct varieties of any one species of grass would have the best chance of succeeding and of increasing in numbers -- thus of supplanting the less distinct varieties.

"In an extremely small area, especially if freely open to immigration -- and where the contest between individual and individual must be very severe -- we always find great diversity in its inhabitants.

"For instance, I found that a piece of turf, three feet by four in size, which had been exposed for many years to exactly the same conditions, supported twenty species of plants. These belonged to eighteen genera and to eight orders, which shows how much these plants differed from each other.

"So it is with the plants and insects on small and uniform islets -- also in small ponds of fresh water.

"It might have been expected that the plants which would succeed in becoming naturalised in any land would generally have been closely allied to the indigenes. These are commonly looked at as specially created and adapted for their own country.

"It might also, perhaps, have been expected that naturalised plants would have belonged to a few groups more especially adapted to certain stations in their new homes.

"But the case is very different.

"Alph. de Candolle has well remarked, in his great and admirable work, that floras gain by naturalisation -- proportionally with the number of the native genera and species -- far more in new genera than in new species.

"To give a single instance. In the last edition of Dr. Asa Gray's Manual of the Flora of the Northern United States, 260 naturalized plants are enumerated, and these belong to 162 genera.

"We thus see that these naturalised plants are of a highly diversified nature. They differ, moreover, to a large extent, from the indigenes, for out of the 162 naturalised genera, no less than 100 genera are not there indigenous.

"We may at least infer that diversification of structure, amounting to new generic differences, would be profitable to them.

"The advantage of diversification of structure in the inhabitants of the same region is, in fact, the same as that of the physiological division of labour in the organs of the same individual body- a subject so well elucidated by Milne Edwards.

"No physiologist doubts that a stomach adapted to digest vegetable matter alone, or flesh alone, draws most nutriment from these substances.

"So in the general economy of any land, the more widely and perfectly the animals and plants are diversified for different habits of life, so will a greater number of individuals be capable of there supporting themselves.

"A set of animals, with their organisation but little diversified, could hardly compete with a set more perfectly diversified in structure.

"Let A to L represent the species of a genus large in its own country.

"These species are supposed to resemble each other in unequal degrees -- as is so generally the case in nature -- and as is represented in the diagram by the letters standing at unequal distances.

"I have said a large genus -- because as we saw in the second chapter -- on an average more species vary in large genera than in small genera. The varying species of the large genera present a greater number of varieties.

"We have, also, seen that the species -- which are the commonest and the most widely diffused -- vary more than do the rare and restricted species.

"Let (A) be a common, widely-diffused, and varying species, belonging to a genus large in its own country. The branching and diverging lines of unequal lengths proceeding from (A), may represent its varying offspring.

"The variations are supposed to be extremely slight, but of the most diversified nature. They are not supposed all to appear simultaneously, but often after long intervals of time -- nor are they an supposed to endure for equal periods.

"Only those variations which are in some way profitable will be preserved or naturally selected.

"Here the importance of the principle of benefit derived from divergence of character comes in. This will generally lead to the most different or divergent variations -- represented by the outer lines -- being preserved and accumulated by natural selection.

"After a thousand generations, species (A) is supposed to have produced two fairly well-marked varieties, namely a1 and m1.

"These two varieties will generally still be exposed to the same conditions which made their parents variable, and the tendency to variability is in itself hereditary. Consequently they will likewise tend to vary, and commonly in nearly the same manner as did their parents.

"Moreover, these two varieties, being only slightly modified forms, will tend to inherit those advantages which made their parent (A) more numerous than most of the other inhabitants of the same country.

"They will also partake of those more general advantages which made the genus to which the parent-species belonged, a large genus in its own country.

"And after this interval, variety a1 is supposed in the diagram to have produced variety a2, which will, owing to the principle of divergence, differ more from (A) than did variety a1.

"Variety m1 is supposed to have produced two varieties, namely m2 and s2, differing from each other, and more considerably from their common parent (A).

"We may continue the process by similar steps for any length of time -- some of the varieties, after each thousand generations, producing only a single variety, but in a more and more modified condition -- some producing two or three varieties -- and some failing to produce any. Thus the varieties or modified descendants of the common parent (A), will generally go on increasing in number and diverging in character.

"And after this interval, variety a1 is supposed in the diagram to have produced variety a2, which will, owing to the principle of divergence, differ more from (A) than did variety a1.

"Variety m1 is supposed to have produced two varieties, namely m2 and s2, differing from each other, and more considerably from their common parent (A).

"We may continue the process by similar steps for any length of time -- some of the varieties, after each thousand generations, producing only a single variety, but in a more and more modified condition -- some producing two or three varieties, and some failing to produce any.

"It is far more probable that each form remains for long periods unaltered, and then again undergoes modification.

"Nor do I suppose that the most divergent varieties are invariably preserved. A medium form may often long endure, and may or may not produce more than one modified descendant.

"For natural selection will always act according to the nature of the places which are either unoccupied or not perfectly occupied by other beings. This will depend on infinitely complex relations.

"But as a general rule, the more diversified in structure the descendants from any one species can be rendered, the more places they will be enabled to seize on -- and the more their modified progeny will increase.

"This is represented in the diagram by the several divergent branches proceeding from (A).

"The modified offspring from the later and more highly improved branches in the lines of descent, will, it is probable, often take the place of, and so destroy, the earlier and less improved branches.

"This is represented in the diagram by some of the lower branches not reaching to the upper horizontal lines.

"In some cases no doubt the process of modification will be confined to a single line of descent. The number of modified descendants will not be increased although the amount of divergent modification may have been augmented.

"This case would be represented in the diagram, if all the lines proceeding from (A) were removed, excepting that from a1 to a10.

"If we suppose the amount of change between each horizontal line in our diagram to be excessively small, these three forms may still be only well-marked varieties.

"But we have only to suppose the steps in the process of modification to be more numerous or greater in amount, to convert these three forms into well-defined or at least into doubtful species.

"Thus the diagram illustrates the steps by which the small differences distinguishing varieties are increased into the larger differences distinguishing species.

"By continuing the same process for a greater number of generations --as shown in the diagram in a condensed and simplified manner -- we get eight species, marked by the letters between a14 and m14, all descended from (A).

"In the diagram I have assumed that a second species (I) has produced, by analogous steps, after ten thousand generations, either two well-marked varieties (w10 and z10) or two species -- according to the amount of change supposed to be represented between the horizontal lines.

"After fourteen thousand generations, six new species, marked by the letters n14 to z14, are supposed to have. been produced.

"In any genus, the species which are already very different in character from each other, will generally tend to produce the greatest number of modified descendants.

"These will have the best chance of seizing on new and widely different places in the polity of nature. Hence in the diagram I have chosen the extreme species (A) -- and the nearly extreme species (I) -- as those which have largely varied, and have given rise to new varieties and species.

"As in each fully stocked country natural selection necessarily acts by the selected form having some advantage in the struggle for life over other forms, there will be a constant tendency in the improved descendants of any one species to supplant and exterminate in each stage of descent their predecessors and their original progenitor.

"For it should be remembered that the competition will generally be most severe between those forms which are most nearly related to each other in habits, constitution, and structure.

"Hence all the intermediate forms between the earlier and later states -- that is between the less and more improved states of the same species, as well as the original parent-species itself -- will generally tend to become extinct.

"So it probably will be with many whole collateral lines of descent, which will be conquered by later and improved lines.

"These two species (A) and (I) were also supposed to be very common and widely diffused species, so that they must originally have had some advantage over most of the other species of the genus.

"Their modified descendants -- fourteen in number at the fourteen-thousandth generation -- will probably have inherited some of the same advantages. They have also been modified and improved in a diversified manner at each stage of descent, so as to have become adapted to many related places in the natural economy of their country.

"It seems, therefore, extremely probable that they will have taken the places of -- and thus exterminated -- not only their parents (A) and (I), but likewise some of the original species which were most nearly related to their parents.

"Hence very few of the original species will have transmitted offspring to the fourteen-thousandth generation.

"Owing to the divergent tendency of natural selection, the extreme amount of difference in character between species a14 and z14 will be much greater than that between the most distinct of the original eleven species.

"But as the original species (I) differed largely from (A), standing nearly at the extreme end of the original genus, the six descendants from (I) will -- owing to inheritance alone -- differ considerably from the eight descendants from (A).

"The two groups, moreover, are supposed to have gone on diverging in different directions. The intermediate species, also -- and this is a very important consideration -- which connected the original species (A) and (I), have all become, excepting (F), extinct, and have left no descendants.

"Hence the six new species descended from (I), and the eight descendants from (A), will have to be ranked as very distinct genera, or even as distinct sub-families.

"Thus it is, as I believe, that two or more genera are produced -- by descent with modification -- from two or more species of the same genus.

"And the two or more parent-species are supposed to be descended from some one species of an earlier genus.

"In this case, its affinities to the other fourteen new species will be of a curious and circuitous nature.

"Being descended from a form which stood between the parent-species (A) and (I) -- now supposed to be extinct and unknown -- it will be in some degree intermediate in character between the two groups descended from these two species.

"But as these two groups have gone on diverging in character from the type of their parents, the new species (F14) will not be directly intermediate between them, but rather between types of the two groups. Every naturalist will be able to call such cases before his mind.

In the diagram, each horizontal line has hitherto been supposed to represent a thousand generations, but each may represent a million or more generations. It may also represent a section of the successive strata of the earth's crust including extinct remains.

"We shall, when we come to our chapter on Geology, have to refer again to this subject. I think we shall then see that the diagram throws light on the affinities of extinct beings, which -- though generally belonging to the same orders, families, or genera, with those now living -- yet are often, in some degree, intermediate in character between existing groups.

"If, in the diagram, we suppose the amount of change -- represented by each successive group of diverging lines to be great -- the forms marked a14 to p14, those marked b14 and f14, and those marked o14 to m14, will form three very distinct genera.

"We shall also have two very distinct genera descended from (I), differing widely from the descendants of (A). These two groups of genera will thus form two distinct families, or orders, according to the amount of divergent modification supposed to be represented in the diagram.

"This, indeed, might have been expected. As natural selection acts through one form having some advantage over other forms in the struggle for existence, it will chiefly act on those which already have some advantage.

"The largeness of any group shows that its species have inherited from a common ancestor some advantage in common. Hence, the struggle for the production of new and modified descendants will mainly lie between the larger groups which are all trying to increase in number.

"One large group will slowly conquer another large group, reduce its numbers, and thus lessen its chance of further variation and improvement. Within the same large group, the later and more highly perfected sub-groups -- from branching out and seizing on many new places in the polity of Nature -- will constantly tend to supplant and destroy the earlier and less improved sub-groups.

"Small and broken groups and sub-groups will finally disappear.

"Looking to the future, we can predict that the groups of organic beings which are now large and triumphant, and which are least broken up -- that is, which have as yet suffered least extinction -- will, for a long period, continue to increase.

"Which groups will ultimately prevail, no man can predict. We know that many groups, formerly most extensively developed, have now become extinct.

"Looking still more remotely to the future, we may predict that -- owing to the continued and steady increase of the larger groups -- a multitude of smaller groups will become utterly extinct, and leave no modified descendants. Of the species living at any one period, extremely few will transmit descendants to a remote futurity.

"I shall have to return to this subject in the chapter on Classification.

"I may add that as, according to this view, extremely few of the more ancient species have transmitted descendants to the present day, and -- as all the descendants of the same species form a class -- we can understand how it is that there exist so few classes in each main division of the animal and vegetable kingdoms.

"The ultimate result is that each creature tends to become more and more improved in relation to its conditions.

"This improvement inevitably leads to the gradual advancement of the organisation of the greater number of living beings throughout the world.

"But here we enter on a very intricate subject, for naturalists have not defined to each other's satisfaction what is meant by an advance in organisation. Amongst the vertebrata the degree of intellect and an approach in structure to man clearly come into play.

"It might be thought that the amount of change which the various parts and organs pass through in their development from the embryo to maturity would suffice as a standard of comparison. But there are cases -- as with certain parasitic crustaceans -- in which several parts of the structure become less perfect, so that the mature animal cannot be called higher than its larva.

"Von Baer's standard seems the most widely applicable and the best -- namely, the amount of differentiation of the parts of the same organic being -- in the adult state as I should be inclined to add -- and their specialisation for different functions. As Milne Edwards would express it, the completeness of the division of physiological labour.

"But we shall see how obscure this subject is if we look, for instance, to fishes, amongst which some naturalists rank those as highest which, like the sharks, approach nearest to amphibians.

"Other naturalists rank the common bony or teleostean fishes as the highest, inasmuch as they are most strictly fish-like and differ most from the other vertebrate classes.

"All physiologists admit that the specialisation of organs -- inasmuch as in this state they perform their functions better -- is an advantage to each being.

"Hence the accumulation of variations tending towards specialisation is within the scope of natural selection.

"On the other hand, we can see -- bearing in mind that all organic beings are striving to increase at a high ratio and to seize on every unoccupied or less well occupied place in the economy of nature -- that it is quite possible for natural selection gradually to fit a being to a situation in which several organs would be superfluous or useless.

"In such cases there would be retrogression in the scale of organisation.

"And how is it that in each great class some forms are far more highly developed than others?

"Why have not the more highly developed forms everywhere supplanted and exterminated the lower?

"Lamarck -- who believed in an innate and inevitable tendency towards perfection in all organic beings -- seems to have felt this difficulty so strongly, that he was led to suppose that new and simple forms are continually being produced by spontaneous generation. Science has not as yet proved the truth of this belief, whatever the future may reveal.

"On our theory the continued existence of lowly organisms offers no difficulty. For natural selection, or the survival of the fittest, does not necessarily include progressive development- it only takes advantage of such variations as arise and are beneficial to each creature under its complex relations of life.

"It may be asked what advantage, as far as we can see, would it be to an infusorian animalcule- to an intestinal worm- or even to an earthworm, to be highly organised.

"If it were no advantage, these forms would be left -- by natural selection, unimproved or but little improved -- and might remain for indefinite ages in their present lowly condition.

"Geology tells us that some of the lowest forms, as the infusoria and rhizopods, have remained for an enormous period in nearly their present state.

"But mammals and fish hardly come into competition with each other. The advancement of the whole class of mammals -- or of certain members in this class -- to the highest grade would not lead to their taking the place of fishes.

"Physiologists believe that the brain must be bathed by warm blood to be highly active, and this requires aerial respiration. So that warm-blooded mammals when inhabiting the water lie under a disadvantage in having to come continually to the surface to breathe.

"With fishes, members of the shark family would not tend to supplant the lancelet. For the lancelet, as I hear from Fritz Muller, has as sole companion and competitor on the barren sandy shore of South Brazil, an anomalous annelid.

"The three lowest orders of mammals -- namely, marsupials, edentata, and rodents -- co-exist in South America in the same region with numerous monkeys, and probably interfere little with each other.

"Although organisation, on the whole, may have advanced and be still advancing throughout the world, yet the scale will always present many degrees of perfection. For the high advancement of certain whole classes -- or of certain members of each class -- does not at all necessarily lead to the extinction of those groups with which they do not enter into close competition.

"In some cases variations or individual differences of a favourable nature may never have arisen for natural selection to act on and accumulate.

"In no case, probably, has time sufficed for the utmost possible amount of development. In some few cases there has been what we must call retrogression of organisation.

"Mr. Herbert Spencer would probably answer that -- as soon as simple unicellular organism came by growth or division to be compounded of several cells -- or became attached to any supporting surface -- his law "that homologous units of any order become differentiated in proportion as their relations to incident forces" would come into action.

"But as we have no facts to guide us, speculation on the subject is almost useless.

"It is, however, an error to suppose that there would be no struggle for existence -- and, consequently, no natural selection until many forms had been produced.

"Variations in a single species inhabiting an isolated station might be beneficial. Thus the whole mass of individuals might be modified, or two distinct forms might arise.

"But, as I remarked towards the close of the Introduction, no one ought to feel surprise at much remaining as yet unexplained on the origin of species. Make due allowance for our profound ignorance on the mutual relations of the inhabitants of the world at the present time, and still more so during past ages."

Darwin is now beginning to ask the type of questions that may have upset the population of his period. He talks about the "first dawn of time." He says that all organic beings at that time had just the "simplest structure." That, of course, goes against believers of various religions. But he continues on with that premise and as there must always be a premise introducing an "argument," we will follow along with his.

Wondering what could have been the first steps in advancement from a simple cell, or how it could have taken place, Darwin merely says that speculation is useless as we have no facts about life at that time.

He says, however, that even if all the organisms at that time were just one-celled, there still would have been changes and a "struggle for existence." Environments would have been different and variations would have come into existence to meet the needs of the environments. Thus even the one-celled organisms would have ended up being different. Or a certain new variety would have gradually have become radically different from the others and, in effect, have become a new specie.

All this being possibly so, he warns us to be cautious and remind ourselves that not only do we know little about relationships between organisms currently in existence, but obviously much much less about organisms that existed "way back there" and are no longer with us.

"If two species -- belonging to two distinct though allied genera -- had both produced a large number of new and divergent forms, it is conceivable that these might approach each other so closely that they would have all to be classed under the same genus. Thus the descendants of two distinct genera would converge into one.

"But it would in most cases be extremely rash to attribute to convergence a close and general similarity of structure in the modified descendants of widely distinct forms.

"The shape of a crystal is determined solely by the molecular forces. It is not surprising that dissimilar substances should sometimes assume the same form.

"With organic beings we should bear in mind that the form of each depends on an infinitude of complex relations -- namely on the variations which have arisen -- these being due to causes far too intricate to be followed out on the nature of the variations which have been preserved or selected.

"This depends on the surrounding physical conditions, and in a still higher degree on the surrounding organisms with which each being has come into competition.

"Lastly, on inheritance (in itself a fluctuating element) from innumerable progenitors, all of which have had their forms determined through equally complex relations.

"It is incredible that the descendants of two organisms -- which had originally differed in a marked manner -- should ever afterwards converge so closely as to lead to a near approach to identity throughout their whole organisation.

"As far as mere inorganic conditions are concerned, it seems probable that a sufficient number of species would soon become adapted to all considerable diversities of heat, moisture, &c..

"But I fully admit that the mutual relations of organic beings are more important. As the number of species in any country goes on increasing, the organic conditions of life must become more and more complex.

"Consequently there seems at first sight no limit to the amount of profitable diversification of structure, and therefore no limit to the number of species which might be produced.

"We do not know that even the most prolific area is fully stocked with specific forms. At the Cape of Good Hope and in Australia, which support such an astonishing number of species, many European plants have become naturalised.

"But geology shows us, that from an early part of the tertiary period the number of species of shells and that from the middle part of this same period the number of mammals, has not greatly or at all increased.

"What then checks an indefinite increase in the number of species? The amount of life (I do not mean the number of specific forms) supported on an area must have a limit, depending so largely as it does on physical conditions.

"Therefore, if an area be inhabited by very many species, each or nearly each species will be represented by few individuals; and such species will be liable to extermination from accidental fluctuations in the nature of the seasons or in the number of their enemies.

"The process of extermination in such cases would be rapid, whereas the production of new species must always be slow.

"Imagine the extreme case of as many species as individuals in England, and the first severe winter or very dry summer would exterminate thousands on thousands of species.

"Rare species -- and each species will become rare if the number of species in any country becomes indefinitely increased -- will, on the principle often explained, present within a given period few favourable variations. Consequently, the process of giving birth to new specific forms would thus be retarded.

"When any species becomes very rare, close interbreeding will help to exterminate it. Authors have thought that this comes into play in accounting for the deterioration of the aurochs in Lithuania, of red deer in Scotland, and of bears in Norway, &e.

"Lastly, and this I am inclined to think is the most important element -- a dominant species, which has already beaten many competitors in its own home, will tend to spread and supplant many others. Alph. de Candolle has shown that those species which spread widely, tend generally to spread very widely. Consequently, they will tend to supplant and exterminate several species in several areas, and thus check the inordinate increase of specific forms throughout the world.

"But if variations useful to any organic being ever do occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life. From the strong principle of inheritance, these will tend to produce offspring similarly characterised.

"This principle of preservation, or the survival of the fittest, I have called Natural Selection.

"It leads to the improvement of each creature in relation to its organic and inorganic conditions of life, and consequently, in most cases, to what must be regarded as an advance in organisation.

"Amongst many animals, sexual selection will have given its aid to ordinary selection, by assuring to the most vigorous and best adapted males the greatest number of offspring.

"But we have already seen how it entails extinction. How largely extinction has acted in the world's history, geology plainly declares.

"Natural selection also leads to divergence of character. The more organic beings diverge in structure, habits, and constitution, by so much the more can a large number be supported on the area. We see proof by looking to the inhabitants of any small spot, and to the productions naturalised in foreign lands.

"Therefore, during the modification of the descendants of any one species, and during the incessant struggle of all species to increase in numbers, the more diversified the descendants become, the better will be their chance of success in the battle for life.

"These tend to transmit to their modified offspring that superiority which now makes them dominant in their own countries. Natural selection, as has just been remarked, leads to divergence of character and to much extinction of the less improved and intermediate forms of life.

"On these principles, the nature of the affinities, and the generally well-defined distinctions between the innumerable organic beings in each class throughout the world, may be explained.

"It is a truly wonderful fact- the wonder of which we are apt to overlook from familiarity- that all animals and all plants throughout all time and space should be related to each other in groups, subordinate to groups -- in the manner which we everywhere behold -- namely, varieties of the same species most closely related -- species of the same genus less closely and unequally related -- forming sections and sub-genera, species of distinct genera much less closely related -- and genera related in different degrees, forming sub-families, families, orders, sub-classes and classes.

"The several subordinate groups in any class cannot be ranked in a single file -- but seem clustered round points, and these round other points -- and so on in almost endless cycles.

"If species had been independently created, no explanation would have been possible of this kind of classification.

"The green and budding twigs may represent existing species.

"Those produced during former years may represent the long succession of extinct species.

"At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches. In the same manner species and groups of species have at all times overmastered other species in the great battle for life.

"The limbs, divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was young, budding twigs. This connection of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups.

"Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear the other branches. So with the species which lived during long-past geological periods very few have left living and modified descendants.

"From the first growth of the tree, many a limb and branch has decayed and dropped off. These fallen branches of various sizes may represent those whole orders, families, and genera which have now no living representatives, and which are known to us only in a fossil state.

"As we here and there see a thin straggling branch springing from a fork low down in a tree, and which by some chance has been favoured and is still alive on its summit, so we occasionally see an animal like the Ornithorhynchus or Lepidosiren, which in some small degree connects by its affinities to large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station.

"There is reason to believe that in the course of time the effects have been greater than can be proved by clear evidence.

"But we may safely conclude that the innumerable complex co-adaptations of structure, which we see throughout nature between various organic beings, cannot be attributed simply to such action.

"In the following cases the conditions seem to have produced some slight definite effect.

"E. Forbes asserts that shells at their southern limit -- and when living in shallow water -- are more brightly coloured than those of the same species from further north or from a greater depth. But this certainly does not always hold good.

"Mr. Gould believes that birds of the same species are more brightly coloured under a clear atmosphere, than when living near the coast or on islands.

"Wollaston is convinced that residence near the sea affects the colours of insects.

"Moquin-Tandon gives a list of plants which, when growing near the sea-shore, have their leaves in some degree fleshy, though not elsewhere fleshy.

"Again, innumerable instances are known to every naturalist, of species keeping true, or not varying at all, although living under the most opposite climates.

"Such considerations as these incline me to lay less weight on the direct action of the surrounding conditions, than on a tendency to vary, due to causes of which we are quite ignorant.

"In one sense the conditions of life may be said -- not only to cause variability, either directly or indirectly, but likewise to include natural selection -- for the conditions determine whether this or that variety shall survive.

"But when man is the selecting agent, we clearly see that the two elements of change are distinct. Variability is in some manner excited, but it is the will of man which accumulates the variations in certain directions.

"Under free nature, we have no standard of comparison, by which to judge of the effects of long-continued use or disuse. We know not the parent-forms. But many animals possess structures which can be best explained by the effects of disuse.

"As Professor Owen has remarked, there is no greater anomaly in nature than a bird that cannot fly; yet there are several in this state. The logger-headed duck of South America can only flap along the surface of the water -- and has its wings in nearly the same condition as the domestic Aylesbury duck. It is a remarkable fact that the young birds, according to Mr. Cunningham, can fly, while the adults have lost this power.

"As the larger ground-feeding birds seldom take flight except to escape danger, it is probable that the nearly wingless condition of several birds -- now inhabiting or which lately inhabited several oceanic islands, tenanted by no beast of prey -- has been caused by disuse.

"The ostrich indeed inhabits continents, and is exposed to danger from which it cannot escape by flight -- but it can defend itself by kicking its enemies, as efficiently as many quadrupeds.

"In the Onites apelles the tarsi are so habitually lost, that the insect has been described as not having them.

"In some other genera they are present, but in a rudimentary condition.

"In the Ateuchus, or sacred beetle of the Egyptians, they are totally deficient.

"The evidence that accidental mutilations can be inherited is at present not decisive. But the remarkable cases observed by Brown-Sequard in guinea-pigs, of the inherited effects of operations, should make us cautious in denying this tendency.

"Mr. Wollaston has discovered the remarkable fact that 200 beetles, out of the 550 species (but more are now known) inhabiting Madeira, are so far deficient in wings that they cannot fly. That, of the twenty-nine endemic genera, no less than twenty-three have all their species in this condition!

"Several facts, namely, that beetles in many parts of the world are frequently blown to sea and perish. That the beetles in Madeira, as observed by Mr. Wollaston, lie much concealed, until the wind lulls and the sun shines.

"That the proportion of wingless beetles is larger on the exposed Desertas than in Madeira itself.

"Especially the extraordinary fact, so strongly insisted on by Mr. Wollaston, that certain large groups of beetles -- elsewhere excessively numerous, which absolutely require the use of their wings -- are here almost entirely absent.

"These several considerations make me believe that the wingless condition of so many Madeira beetles is mainly due to the action of natural selection, combined probably with disuse.

"For when a new insect first arrived on the island, the tendency of natural selection to enlarge or to reduce the wings, would depend on whether a greater number of individuals were saved by successfully battling with the winds -- or by giving up the attempt and rarely or never flying.

"This state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection.

"In South America, a burrowing rodent, the tucotuco, or Ctenomys, is even more subterranean in its habits than the mole. I was assured by a Spaniard, who had often caught them, that they were frequently blind.

"One which I kept alive was certainly in this condition, the cause, as appeared on dissection, having been inflammation of the nictitating membrane. As frequent inflammation of the eyes must be injurious to any animal -- and as eyes are certainly not necessary to animals having subterranean habits -- a reduction in their size, with the adhesion of the eyelids and growth of fur over them, might in such case be an advantage. If so, natural selection would aid the effects of disuse.

"It is well known that several animals, belonging to the most different classes, which inhabit the caves of Carniola and of Kentucky, are blind. In some of the crabs the foot-stalk for the eye remains, though the eye is gone -- the stand for the telescope is there, though the telescope with its glasses has been lost.

"As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, their loss may be attributed to disuse.

"So that -- in accordance with the old view of the blind animals having been separately created for the American and European caverns -- very close similarity in their organisation and affinities might have been expected.

"This is certainly not the case if we look at the two whole faunas.

"With respect to the insects alone, Schiodte has remarked, "We are accordingly prevented from considering the entire phenomenon in any other light than something purely local -- and the similarity which is exhibited in a few forms between the Mammoth cave (in Kentucky) and the caves in Carniola -- otherwise than as a very plain expression of that analogy which subsists generally between the fauna of Europe and of North America."

"On my view we must suppose that American animals, having in most cases ordinary powers of vision, slowly migrated by successive generations from the outer world into the deeper and deeper recesses of the Kentucky caves, as did European animals into the caves of Europe.

"We have some evidence of this gradation of habit.

"As Schiodte remarks, 'We accordingly look upon the subterranean faunas as small ramifications which have penetrated into the earth from the geographically limited faunas of the adjacent tracts -- and which, as they extended themselves into darkness, have been accommodated to surrounding circumstances. Animals not far remote from ordinary forms, prepare the transition from light to darkness. Next follow those that are constructed for twilight; and, last of all, those destined for total darkness, and whose formation is quite peculiar.'

"These remarks of Schiodte's it should be understood, apply not to the same, but to distinct species.

"By the time that an animal had reached -- after numberless generations -- the deepest recesses, disuse will on this view have more or less perfectly obliterated its eyes. Natural selection will often have effected other changes, such as an increase in the length of the antennae or palpi, as a compensation for blindness.

"Notwithstanding such modifications, we might expect still to see in the cave-animals of America, affinities to the other inhabitants of that continent -- and in those of Europe to the inhabitants of the European continent.

"And this is the case with some of the American cave-animals, as I hear from Professor Dana. Some, of the European cave insects are very closely allied to those of the surrounding country. It would be difficult to give any rational explanation of the affinities of the blind cave-animals to the other inhabitants of the two continents on the ordinary view of their independent creation.

"That several of the inhabitants of the caves of the Old and New Worlds should be closely related, we might expect from the well-known relationship of most of their other productions. As a blind species of Bathyscia is found in abundance on shady rocks far from caves, the loss of vision in the cave-species of this one genus has probably had no relation to its dark habitation.

"It is natural that an insect already deprived of vision should readily become adapted to dark caverns. Another blind genus (Anophthaimus) offers this remarkable peculiarity, that the species, as Mr. Murray observes, have not as yet been found anywhere except in caves.

"Yet those which inhabit the several eaves of Europe and America are distinct. It is possible that the progenitors of these several species, whilst they were furnished with eyes, may formerly have ranged over both continents -- and then have become extinct -- excepting in their present secluded abodes.

"As it is extremely common for distinct species belonging to the same genus to inhabit hot and cold countries -- if it be true that all the species of the same genus are descended from a single parent-form, acclimatisation must be readily effected during a long course of descent.

"It is notorious that each species is adapted to the climate of its own home. Species from an arctic or even from a temperate region cannot endure a tropical climate, or conversely. So again, many succulent plants cannot endure a damp climate.

"But the degree of adaptation of species to the climates under which they live is often overrated. We may infer this from our frequent inability to predict whether or not an imported plant will endure our climate -- and from the number of plants and animals brought from different countries which are here perfectly healthy.

"We have reason to believe that species in a state of nature are closely limited in their ranges by the competition of other organic beings quite as much as, or more than, by adaptation to particular climates. But whether or not this adaptation is in most cases very close, we have evidence with some few plants, of their becoming -- to a certain extent -- naturally habituated to different temperatures.

"They become acclimatised.

"Thus the pines and rhododendrons -- raised from seed collected by Dr. Hooker from the same species growing at different heights on the Himalaya -- were found to possess in this country different constitutional powers of resisting cold.

"Mr. Thwaites informs me that he has observed similar facts in Ceylon. Analogous observations have been made by Mr. H. C. Watson on European species of plants brought from the Azores to England. I could give other cases.

"In regard to animals, several authentic instances could be adduced of species having largely extended -- within historical times -- their range from warmer to cooler latitudes -- and conversely.

"That habit or custom has some influence, I must believe, both from analogy and from the incessant advice given in agricultural works, even in the ancient encyclopaedias of China -- to be very cautious in transporting animals from one district to another.

"And as it is not likely that man should have succeeded in selecting so many breeds and sub-breeds with constitutions specially fitted for their own districts, the result must, I think, be due to habit.

"On the other hand, natural selection would inevitably tend to preserve those individuals which were born with constitutions best adapted to any country which they inhabited.

"In treatises on many kinds of cultivated plants, certain varieties are said to withstand certain climates better than others. This is strikingly shown in works on fruit-trees published in the United States, in which certain varieties are habitually recommended for the northern and others for the southern States. As most of these varieties are of recent origin, they cannot owe their constitutional differences to habit. The case of the Jerusalem artichoke -- which is never propagated in England by seed, and of which consequently new varieties have not been produced -- has even been advanced, as proving that acclimatisation cannot be effected, for it is now as tender as ever it was! The case, also, of the kidney-bean has been often cited for a similar purpose, and with much greater weight. Until someone will sow, during a score of generations, his kidney-beans so early that a very large proportion are destroyed by frost -- and then collect seed from the few survivors, with care to prevent accidental crosses -- and then again get seed from these seedlings, with the same precautions -- the experiment cannot be said to have been tried.

"That habit or custom has some influence, I must believe -- both from analogy and from the incessant advice given in agricultural works, even in the ancient encyclopaedias of China -- to be very cautious in transporting animals from one district to another.

"And as it is not likely that man should have succeeded in selecting so many breeds and sub-breeds with constitutions specially fitted for their own districts, the result must, I think, be due to habit.

"On the other hand, natural selection would inevitably tend to preserve those individuals which were born with constitutions best adapted to any country which they inhabited.

"In treatises on many kinds of cultivated plants, certain varieties are said to withstand certain climates better than others. This is strikingly shown in works on fruit-trees published in the United States -- in which certain varieties are habitually recommended for the northern and others for the southern States. As most of these varieties are of recent origin, they cannot owe their constitutional differences to habit.

"The case of the Jerusalem artichoke -- which is never propagated in England by seed, and of which consequently new varieties have not been produced -- has even been advanced, as proving that acclimatisation cannot be effected. It is now as tender as ever it was!

"The case, also, of the kidney-bean has been often cited for a similar purpose, and with much greater weight. Until someone will sow, during a score of generations, his kidney-beans so early that a very large proportion are destroyed by frost -- and then collect seed from the few survivors, with care to prevent accidental crosses -- and then again get seed from these seedlings, with the same precautions -- the experiment cannot be said to have been tried.

"This is a very important subject, most imperfectly understood. No doubt wholly different classes of facts may be here easily confounded together.

"We shall presently see that simple inheritance often gives the false appearance of correlation.

"One of the most obvious real cases is, that variations of structure arising in the young or larvae naturally tend to affect the structure of the mature animal.

"The several parts of the body which are homologous -- and which, at an early embryonic period, are identical in structure -- and which are necessarily exposed to similar conditions, seem eminently liable to vary in a like manner.

"We see this in the right and left sides of the body varying in the same manner -- in the front and hind legs -- and even in the jaws and limbs, varying together. The lower jaw is believed by some anatomists to be homologous with the limbs.

"Hard parts seem to affect the form of adjoining soft parts. It is believed by some authors that with birds the diversity in the shape of the pelvis causes the remarkable diversity in the shape of their kidneys.

"Others believe that the shape of the pelvis in the human mother influences by pressure the shape of the head of the child.

"Isidore Geoffroy St-Hilaire has forcibly remarked that certain malconformations frequently, and that others rarely, co-exist, without our being able assign any reason.

"What can be more singular than the relation in cats between complete whiteness and blue eyes with deafness -- or between the tortoise-shell colour and the female sex -- or in pigeons between their feathered feet and skin betwixt the outer toes -- or between the presence of more or less down on the young pigeon when first hatched, with the future colour of its plumage.

"Or, again, the relation between the hair and teeth in the naked Turkish dog, though here no doubt homology comes into play?

"With respect to this latter case of correlation, I think it can hardly be accidental -- that the two orders of mammals which are most abnormal in their dermal covering, viz., Cetacea (whales) and Edentata (armadilloes, scaly ant-eaters, &c.,) are likewise on the whole the most abnormal in their teeth.

"Every one is familiar with the difference between the ray and central florets of, for instance, the daisy. This difference is often accompanied with the partial or complete abortion of the reproductive organs.

"But in some of these plants, the seeds also differ in shape and sculpture. These differences have sometimes been attributed to the pressure of the involuera on the florets, or to their mutual pressure. The shape of the seeds in the ray-florets of some Compositae countenances this idea.

"But with the Umbelliferae, it is by no means, as Dr. Hooker informs me, the species with the densest heads which most frequently differ in their inner and outer flowers.

"It might have been thought that the development of the ray-petals by drawing nourishment from the reproductive organs causes their abortion. But this can hardly be the sole cause, for in some Compositae the seeds of the outer and inner florets differ -- without any difference in the corolla.

"Possibly these several differences may be connected with the different flow of nutriment towards the central and external flowers. We know, at least, that with irregular flowers, those nearest to the axis are most subject to peloria, that is to become abnormally symmetrical.

"I may add -- as an instance of this fact, and as a striking case of correlation -- that in many pelargoniums, the two upper petals in the central flower of the truss often lose their patches of darker colour. When this occurs, the adherent nectary is quite aborted -- the central flower thus becoming peloric or regular.

"But with respect to the seeds, it seems impossible that their differences in shape, which are not always correlated with any difference in the corolla, can be in any way beneficial. Yet in the Umbelliferae these differences are of such apparent importance-- the seeds being sometimes orthospermous in the exterior flowers and coelospermous in the central flowers -- that the elder De Candolle founded his main divisions in the order on such characters.

"An ancient progenitor may have acquired through natural selection some one modification in structure -- and, after thousands of generations, some other and independent modification -- and these two modifications -- having been transmitted to a whole group of descendants with diverse habits -- would naturally be thought to be in some necessary manner correlated.

"Some other correlations are apparently due to the manner in which natural selection can alone act.

"I think this holds true to a certain extent with our domestic productions. If nourishment flows to one part or organ in excess, it rarely flows, at least in excess, to another part. Thus it is difficult to get a cow to give much milk and to fatten readily.

"The same varieties of the cabbage do not yield abundant and nutritious foliage and a copious supply of oil-bearing seeds. When the seeds in our fruits become atrophied, the fruit itself gains largely in size and quality.

"In our poultry, a large tuft of feathers on the head is generally accompanied by a diminished comb -- and a large beard by diminished wattles.

"With species in a state of nature it can hardly be maintained that the law is of universal application. But many good observers, more especially botanists, believe in its truth.

"It is hopeless to attempt to convince any one of the truth of the above proposition without giving the long array of facts which I have collected, and which cannot possibly be here introduced. I can only state my conviction that it is a rule of high generality.

"I am aware of several causes of error, but I hope that I have made due allowance for them.

"It should be understood that the rule by no means applies to any part, however unusually developed, unless it be unusually developed in one species or in a few species in comparison with the same part in many closely allied species. Thus, the wing of a bat is a most abnormal structure in the class of mammals, but the rule would not apply here, because the whole group of bats possesses wings. It would apply only if some one species had wings developed in a remarkable manner in comparison with the other species of the same genus.

"The rule applies very strongly in the case of secondary sexual characters, when displayed in any unusual manner. The term, secondary sexual characters, used by Hunter, relates to characters which are attached to one sex, but are not directly connected with the act of reproduction. The rule applies to males and females -- but more rarely to the females, as they seldom offer remarkable secondary sexual characters.

"The rule being so plainly applicable in the case of secondary sexual characters may be due to the great variability of these characters -- whether or not displayed in any unusual manner -- of which fact I think there can be little doubt.

"But that our rule is not confined to secondary sexual characters is clearly shown in the case of hermaphrodite cirripedes.

"I particularly attended to Mr. Waterhouse's remark, whilst investigating this Order, and I am fully convinced that the rule almost always holds good.

"I shall, in a future work, give a list of all the more remarkable cases. I will here give only one, as it illustrates the rule in its largest application.

"The opereular valves of sessile cirripedes (rock barnacles) are, in every sense of the word, very important structures. They differ extremely little even in distinct genera.

"I have particularly attended to them.. The rule certainly seems to hold good in this class.

"I cannot make out that it applies to plants, and this would have seriously shaken my belief in its truth, had not the great variability in plants made it particularly difficult to compare their relative degrees of variability.

"When we see any part or organ developed in a remarkable degree or manner in a species, the fair presumption is that it is of high importance to that species. Nevertheless it is in this case eminently liable to variation.

"Why should this be so? On the view that each species has been independently created, with all its parts as we now see them, I can see no explanation. But on the view that groups of species are descended from some other species -- and have been modified through natural selection -- I think we can obtain some light.

"First let me make some preliminary remarks. If, in our domestic animals, any part or the whole animal be neglected -- and no selection be applied -- that part (for instance, the comb in the Dorking fowl) or the whole breed will cease to have a uniform character. The breed may be said to be degenerating.

"In rudimentary organs -- and in those which have been but little specialised for any particular purpose, and perhaps in polymorphic groups -- we see a nearly parallel case. In such cases natural selection either has not or cannot have come into full play, and thus the organisation is left in a fluctuating condition.

"But what here more particularly concerns us is -- that those points in our domestic animals, which at the present time are undergoing rapid change by continued selection -- are also eminently liable to variation.

"Look at the individuals of the same breed of the pigeon. See what a prodigious amount of difference there is in the beaks of tumblers, in the beaks and wattle of carriers, in the carriage and tail of fantails, &c., these being the points now mainly attended to by English fanciers.

"Even in the same sub-breed, as in that of the short-faced tumbler, it is notoriously difficult to breed nearly perfect birds, many departing widely from the standard. There may truly be said to be a constant struggle going on between -- on the one hand, the tendency to reversion to a less perfect state, as well as an innate tendency to new variations -- and, on the other hand, the power of steady selection to keep the breed true.

"In the long run selection gains the day, and we do not expect to fail so completely as to breed a bird as coarse as a common tumbler pigeon from a good short-faced strain.

"This period will seldom be remote in any extreme degree, as species rarely endure for more than one geological period.

"An extraordinary amount of modification implies an unusually large and long-continued amount of variability -- which has continually been accumulated by natural selection for the benefit of the species.

"But as the variability of the extraordinarily developed part or organ has been so great and long-continued within a period not excessively remote, we might, as a general rule, still expect to find more variability in such parts than in other parts of the organisation which have remained for a much longer period nearly constant.

"This, I am convinced, is the case.

"That the struggle between natural selection on the one hand, and the tendency to reversion and variability on the other hand, will in the course of time cease. That the most abnormally developed organs may be made constant, I see no reason to doubt.

"Hence, when an organ -- however abnormal it may be -- has been transmitted in approximately the same condition to many modified descendants -- as in the case of the wing of the bat -- it must have existed, according to our theory, for an immense period in nearly the same state. Thus it has come not to be more variable than any other structure.

"It is only in those cases in which the modification has been comparatively recent and extraordinarily great that we ought to find the generative variability, as it may be called, still present in a high degree.

"It is notorious that specific characters are more variable than generic.

"To explain by a simple example what is meant -- if in a large genus of plants some species had blue flowers and some had red -- the colour would be only a specific character -- and no one would be surprised at one of the blue species varying into red, or conversely. But if all the species had blue flowers, the colour would become a generic character, and its variation would be a more unusual circumstance.

"I have chosen this example because the explanation which most naturalists would advance is not here applicable -- namely, that specific characters are more variable than generic -- because they are taken from parts of less physiological importance than those commonly used for classing genera.

"I believe this explanation is partly, yet only indirectly, true. I shall, however, have to return to this point in the chapter on Classification.

"It would be almost superfluous to adduce evidence in support of the statement -- that ordinary specific characters are more variable than generic.

"But with respect to important characters I have repeatedly noticed in works on natural history, that when an author remarks with surprise that some important organ or part -- which is generally very constant throughout a large group of species -- differs considerably in closely-allied species, it is often variable in the individuals of the same species.

"And this fact shows that a character -- which is generally of generic value, when it sinks in value and becomes only of specific value -- often becomes variable, though its physiological importance may remain the same.

"I think it will be admitted by naturalists, without my entering on details, that secondary sexual characters are highly variable.

"It will also be admitted that species of the same group differ from each other more widely in their secondary sexual characters, than in other parts of their organisation.

"Compare, for instance, the amount of difference between the males of gallinaceous birds -- in which secondary sexual characters are strongly displayed -- with the amount of difference between the females.

"The cause of the original variability of these characters is not manifest.

"But we can see why they should not have been rendered as constant and uniform as others. They are accumulated by sexual selection -- which is less rigid in its action than ordinary selection, as it does not entail death -- but only gives fewer off-spring to the less favoured males.

"Of this fact I will give in illustration the two first instances which happen to stand on my list. As the differences in these cases are of a very unusual nature, the relation can hardly be accidental.

"The same number of joints in the tarsi is a character common to very large groups of beetles. In the Engidoe, as Westwood has remarked, the number varies greatly. The number likewise differs in the two sexes of the same species.

"Again in the fossorial hymenoptera, the neuration of the wings is a character of the highest importance, because common to large groups. But in certain genera the neuration differs in the different species, and likewise in the two sexes of the same species.

"Sir J. Lubbock has recently remarked, that several minute crustaceans offer excellent illustrations of this law. 'In Pontella, for instance, the sexual characters are afforded mainly by the anterior antennae and by the fifth pair of legs: the specific differences also are principally given by these organs.'

"This relation has a clear meaning on my view. I look at all the species of the same genus as having as certainly descended from a common progenitor -- as have the two sexes of any one species.

"A Variety of one Species often assumes a Character proper to an Allied Species -- or reverts to some of the Characters of an early Progenitor.

"These propositions will be most readily understood by looking to our domestic races. The most distinct breeds of the pigeon -- in countries widely apart -- present sub-varieties with reversed feathers on the head, and with feathers on the feet,- characters not possessed by the aboriginal rock-pigeon.

"These then are analogous variations in two or more distinct races.

"The frequent presence of fourteen or even sixteen tail-feathers in the pouter may be considered as a variation representing the normal structure of another race, the fan-tail.

"I presume that no one will doubt that all such analogous variations are due to the several races of the pigeon having inherited from a common parent the same constitution and tendency to variation, when acted on by similar unknown influences.

"In the vegetable kingdom we have a case of analogous variation, in the enlarged stems -- or as commonly called roots -- of the Swedish turnip and Rutabaga, plants which several botanists rank as varieties produced by cultivation from a common parent.

"If this be not so, the case will then be one of analogous variation in two so-called distinct species. To these a third may be added, namely, the common turnip.

"According to the ordinary view of each species having been independently created, we should have to attribute this similarity in the enlarged stems of these three plants, not to the vera causa of community of descent -- and a consequent tendency to vary in a like manner -- but to three separate yet closely related acts of creation. Many similar cases of analogous variation have been observed by Naudin in the great gourd-family, and by various authors in our cereals.

"As all these marks are characteristic of the parent rock-pigeon, I presume that no one will doubt that this is a case of reversion, and not of a new yet analogous variation appearing in the several breeds.

"But when a breed has been crossed only once by some other breed, the offspring occasionally show for many generations a tendency to revert in character to the foreign breed- some say, for a dozen or even a score of generations.

"After twelve generations, the proportion of blood, to use a common expression, from one ancestor, is only 1 in 2048. Yet, as we see, it is generally believed that a tendency to reversion is retained by this remnant of foreign blood.

"In a breed which has not been crossed -- but in which both parents have lost some character which their progenitor possessed -- the tendency, whether strong or weak, to reproduce the lost character might -- as was formerly remarked, for all that we can see to the contrary -- be transmitted for almost any number of generations.

"When a character which has been lost in a breed, reappears after a great number of generations, the most probable hypothesis is -- not that one individual suddenly takes after an ancestor removed by some hundred generations -- but that in each successive generation the character in question has been lying latent, and at last -- under unknown favourable conditions -- is developed.

"With the barb-pigeon, for instance, which very rarely produces a blue bird, it is probable that there is a latent tendency in each generation to produce blue plumage.

"The abstract improbability of such a tendency being transmitted through a vast number of generations, is not greater than that of quite useless or rudimentary organs being similarly transmitted.

"But characters exclusively due to analogous variation would probably be of an unimportant nature. The preservation of all functionally important characters will have been determined through natural selection, in accordance with the different habits of the species.

"It might further be expected that the species of the same genus would occasionally exhibit reversions to long lost characters.

"As, however, we do not know the common ancestors of any natural group, we cannot distinguish between reversionary and analogous characters.

"If, for instance, we did not know that the parent rock-pigeon was not feather-footed or turn-crowned, we could not have told, whether such characters in our domestic breeds were reversions or only analogous variations. We might have inferred that the blue colour was a case of reversion from the number of the markings, which are correlated with this tint, and which would not probably have all appeared together from simple variation.

"More especially we might have inferred this, from the blue colour and the several marks so often appearing when differently coloured breeds are crossed.

"Hence, although under nature it must generally be left doubtful, what cases are reversions to formerly existing characters, and what are new but analogous variations -- yet we ought, on our theory, sometimes to find the varying offspring of a species assuming characters which are already present in other members of the same group.

"A considerable catalogue, also, could be given of forms intermediate between two other forms, which themselves can only doubtfully be ranked as species. This shows, unless all these closely allied forms be considered as independently created species, that they have in varying assumed some of the characters of the others.

"But the best evidence of analogous variations is afforded by parts or organs which are generally constant in character -- but which occasionally vary so as to resemble, in some degree, the same part or organ in an allied species.

"I have collected a long list of such cases. But here, as before, I lie under the great disadvantage of not being able to give them.

"It is a case almost certainly of reversion. The ass sometimes has very distinct transverse bars on its legs, like those on the legs of the zebra. It has been asserted that these are plainest in the foal, and, from inquiries which I have made, I believe this to be true.

"The stripe on the shoulder is sometimes double, and is very variable in length and outline. A white ass, but not an albino, has been described without either spinal or shoulder stripe. These stripes are sometimes very obscure, or actually quite lost, in dark-coloured asses.

"The koulan of Pallas is said to have been seen with a double shoulder-stripe. Mr. Blyth has seen a specimen of the hemionus with a distinct shoulder-stripe, though it properly has none. I have been informed by Colonel Poole that the foals of this species are generally striped on the legs, and faintly on the shoulder.

"Transverse bars on the legs are not rare in duns, mouse-duns, and in one instance in a chestnut a faint shoulder-stripe may sometimes be seen in duns. I have seen a trace in a bay horse.

"The spine is always striped. The legs are generally barred. The shoulder-stripe, which is sometimes double and sometimes treble, is common. The side of the face, moreover, is sometimes striped.

"The stripes are often plainest in the foal and sometimes quite disappear in old horses.

"Colonel Poole has seen both gray and bay kattywar horses striped when first foaled. I have also reason to suspect -- from information given me by Mr. W. W. Edwards -- that with the English race-horse the spinal stripe is much commoner in the foal than in the fullgrown animal.

"I have myself recently bred a foal from a bay mare (offspring of a Turkoman horse and a Flemish mare) by a bay English race-horse. This foal when a week old was marked on its hinder quarters and on its forehead with numerous, very narrow, dark, zebra-like bars, and its legs were feebly striped. All the stripes soon disappeared completely.

"Without here entering on further details, I may state that I have collected cases of leg and shoulder stripes in horses of very different breeds in various countries from Britain to eastern China -- and from Norway in the north to the Malay Archipelago in the south.

"But this view may be safely rejected. It is highly improbable that the heavy Belgian cart-horse, Welsh ponies, Norwegian cobs, the lanky kattywar race, &c., inhabiting the most distant parts of the world, should all have been crossed with one supposed aboriginal stock.

"Now let us turn to the effects of crossing the several species of the horse-genus.

"Rollin asserts, that the common mule from the ass and horse is particularly apt to have bars on its legs. According to Mr. Gosse, in certain parts of the United States about nine out of ten mules have striped legs.

"I once saw a mule with its legs so much striped that any one might have thought that it was a hybrid-zebra. Mr. W. C. Martin, in his excellent treatise on the horse, has given a figure of a similar mule.

"In four coloured drawings, which I have seen -- of hybrids between the ass and zebra -- the legs were much more plainly barred than the rest of the body. In one of them there was a double shoulder-stripe.

"In Lord Morton's famous hybrid -- from a chestnut mare and male quagga -- the hybrid, and even the pure offspring subsequently produced from the same mare by a black Arabian sire, were much more plainly barred across the legs than is even the pure quagga.

"Lastly, and this is another most remarkable case, a hybrid has been figured by Dr. Gray (and he informs me that he knows of a second case) from the ass and the hemionus. This hybrid -- though the ass only occasionally has stripes on its legs and the hemionus has none and has not even a shoulder-stripe -- nevertheless had all four legs barred, and had three short shoulder-stripes, like those on the dun Devonshire and Welsh ponies, and even had some zebra-like stripes on the sides of its face.

"We see several distinct species of the horse-genus becoming, by simple variation, striped on the legs like a zebra, or striped on the shoulders like an ass.

"In the horse we see this tendency strong whenever a dun tint appears- a tint which approaches to that of the general colouring of the other species of the genus. The appearance of the stripes is not accompanied by any change of form or by any other new character.

"We see this tendency to become striped most strongly displayed in hybrids from between several of the most distinct species.

"Now observe the case of the several breeds of pigeons. They are descended from a pigeon -- including two or three sub-species or geographical races -- of bluish colour, with certain bars and other marks. When any breed assumes by simple variation a bluish tint, these bars and other marks invariably reappear -- but without any other change of form or character.

"When the oldest and truest breeds of various colours are crossed, we see a strong tendency for the blue tint and bars and marks to reappear in the mongrels.

"I have stated that the most probable hypothesis to account for the reappearance of very ancient characters, is -- that there is a tendency in the young of each successive generation to produce the long-lost character. That this tendency -- from unknown causes -- sometimes prevails. And we have just seen that in several species of the horse-genus the stripes are either plainer or appear more commonly in the young than in the old.

"Call the breeds of pigeons, some of which have bred true for centuries, species. How exactly parallel is the case with that of the species of the horse-genus!

"Not in one case out of a hundred can we pretend to assign any reason why this or that part has varied. But whenever we have the means of instituting a comparison -- the same laws appear to have acted in producing the lesser differences between varieties of the same species -- and the greater differences between species of the same genus.

"Changed conditions generally induce mere fluctuating variability. Sometimes they cause direct and definite effects. These may become strongly marked in the course of time, though we have not sufficient evidence on this head.

"Habit in producing constitutional peculiarities and use in strengthening and disuse in weakening and diminishing organs, appear in many cases to have been potent in their effects.

"Homologous parts tend to vary in the same manner. Homologous parts tend to cohere.

"Modifications in hard parts and in external parts sometimes affect softer and internal parts.

"When one part is largely developed, perhaps it tends to draw nourishment from the adjoining parts. Every part of the structure which can be saved without detriment will be saved.

"Changes of structure at an early age may affect parts subsequently developed. Many cases of correlated variation, the nature of which we are unable to understand, undoubtedly occur.

"Multiple parts are variable in number and in structure, perhaps arising from such parts not having been closely specialised for any particular function. Their modifications have not been closely checked by natural selection.

"It follows probably from this same cause, that organic beings low in the scale are more variable than those standing higher in the scale -- and which have their whole organisation more specialised.

"Rudimentary organs, from being useless, are not regulated by natural selection, and hence are variable.

"Specific characters -- that is, the characters which have come to differ since the several species of the same genus branched off from a common parent -- are more variable than generic characters -- or those which have long been inherited, and have not differed from this same period.

"In these remarks we have referred to special parts or organs being still variable, because they have recently varied and thus come to differ. We have also seen in the second chapter that the same principle applies to the whole individual. In a district where many species of a genus are found- that is, where there has been much former variation and differentiation -- or where the manufactory of new specific forms has been actively at work -- in that district and amongst these species, we now find, on an average, most varieties.

"Secondary sexual characters are highly variable, and such characters differ much in the species of the same group.

"Variability in the same parts of the organisation has generally been taken advantage of in giving secondary sexual differences to the two sexes of the same species, and specific differences to the several species of the same genus.

"Any part or organ developed to an extraordinary size or in an extraordinary manner -- in comparison with the same part or organ in the allied species -- must have gone through an extraordinary amount of modification since the genus arose.

"Thus we can understand why it should often still be variable in a much higher degree than other parts. Variation is a long-continued and slow process.

"Natural selection will in such cases not as yet have had time to overcome the tendency to further variability and to reversion to a less modified state.

"But when a species with any extraordinarily-developed organ has become the parent of many modified descendants -- which on our view must be a very slow process, requiring long lapse of time -- in this case, natural selection has succeeded in giving a fixed character to the organ -- in however extraordinary a manner it may have been developed.

"Species inheriting nearly the same constitution from a common parent -- and exposed to similar influences -- naturally tend to present analogous variations, or these same species may occasionally revert to some of the characters of their ancient progenitors.

"Some of them are so serious that to this day I can hardly reflect on them without being in some degree staggered. To the best of my judgment, the greater number are only apparent, and those that are real are not, I think, fatal to the theory.

"These difficulties and objections may be classed under the following heads:- First, why, if species have descended from other species by fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion, instead of the species being, as we see them, well defined?

"Secondly, is it possible that an animal having, for instance, the structure and habits of a bat, could have been formed by the modification of some other animal with widely different habits and structure? Can we believe that natural selection could produce, on the one hand, an organ of trifling importance, such as the tail of a giraffe, which serves as a fly-flapper, and, on the other hand, an organ so wonderful as the eye?

"Thirdly, can instincts be acquired and modified through natural selection? What shall we say to the instinct which leads the bee to make cells, and which has practically anticipated the discoveries of profound mathematicians?

"Thus extinction and natural selection go hand in hand.

"Hence, if we look at each species as descended from some unknown form, both the parent and all the transitional varieties will generally have been exterminated by the very process of the formation and perfection of the new form.

"But, as by this theory innumerable transitional forms must have existed, why do we not find them embedded in countless numbers in the crust of the earth?

"It will be more convenient to discuss this question in the chapter on the Imperfection of the Geological Record. I will here only state that I believe the answer mainly lies in the record being incomparably less perfect than is generally supposed.

"Let us take a simple case. In travelling from north to south over a continent, we generally meet at successive intervals with closely allied or representative species, evidently filling nearly the same place in the natural economy of the land.

"These representative species often meet and interlock. As the one becomes rarer and rarer, the other becomes more and more frequent, till the one replaces the other.

"But if we compare these species where they intermingle, they are generally as absolutely distinct from each other in every detail of structure as are specimens taken from the metropolis inhabited by each.

"By my theory these allied species are descended from a common parent. During the process of modification, each has become adapted to the conditions of life of its own region, and has supplanted and exterminated its original parent-form and all the transitional varieties between its past and present states.

"Hence we ought not to expect at the present time to meet with numerous transitional varieties in each region -- though they must have existed there -- and may be embedded there in a fossil condition.

"Geology would lead us to believe that most continents have been broken up into islands even during the later tertiary periods. In such islands distinct species might have been separately formed without the possibility of intermediate varieties existing in the intermediate zones.

"By changes in the form of the land and of climate, marine areas now continuous must often have existed within recent times in a far less continuous and uniform condition than at present.

"But I will pass over this way of escaping from the difficulty. I believe that many perfectly defined species have been formed on strictly continuous areas.

"Hence the neutral territory between two representative species is generally narrow in comparison with the territory proper to each.

"We see the same fact in ascending mountains, and sometimes it is quite remarkable how abruptly -- as Alph. de Candolle has observed -- a common alpine species disappears.

"The same fact has been noticed by E. Forbes in sounding the depths of the sea with the dredge.

"To those who look at climate and the physical conditions of life as the all-important elements of distribution, these facts ought to cause surprise, as climate and height or depth graduate away insensibly.

"But when we bear in mind that almost every species -- even in its metropolis -- would increase immensely in numbers, were it not for other competing species. Nearly all either prey on or serve as prey for others. Each organic being is either directly or indirectly related in the most important manner to other organic beings.

"We see that the range of the inhabitants of any country by no means exclusively depends on insensibly changing physical conditions, but in a large part on the presence of other species -- on which it lives, or by which it is destroyed -- or with which it comes into competition.

"As these species are already defined objects, not blending one into another by insensible gradations, the range of any one species -- depending as does on the range of others -- will tend to be sharply defined.

"If we take a varying species inhabiting a very large area, we shall have to adapt two varieties to two large areas, and a third variety to a narrow intermediate zone.

"The intermediate variety, consequently, will exist in lesser numbers from inhabiting a narrow and lesser area. Practically, as far as I can make out, this rule holds good with varieties in a state of nature.

"I have met with striking instances of the rule in the case of varieties intermediate between well-marked varieties in the genus Balanus.

"It would appear from information given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, that generally, when varieties intermediate between two other forms occur, they are much rarer numerically than the forms which they connect.

"But it is a far more important consideration -- that during the process of further modification, by which two varieties are supposed to be converted and perfected into two distinct species -- the two which exist in larger numbers, from inhabiting larger areas, will have a great advantage over the intermediate variety -- which exists in smaller numbers in a narrow and intermediate zone.

"For forms existing in larger numbers will have a better chance, within any given period, of presenting further favourable variations for natural selection to seize on, than will the rarer forms which exist in lesser numbers.

"Hence, the more common forms, in the race for life, will tend to beat and supplant the less common forms. These will be more slowly modified and improved.

"It is the same principle which, as I believe, accounts for the common species in each country presenting on an average a greater number of well-marked varieties than do the rarer species.

"I may illustrate what I mean by supposing three varieties of sheep to be kept -- one adapted to an extensive mountainous region -- a second to a comparatively narrow, hilly tract -- and a third to the wide plains at the base. The inhabitants are all trying with equal steadiness and skill to improve their stocks by selection.

"The chances in this case will be strongly in favour of the great holders on the mountains or on the plains, improving their breeds more quickly than the small holders on the intermediate narrow, hilly tract.

"First, because new varieties are very slowly formed, for variation is a slow process.

"Natural selection can do nothing until favourable individual differences or variations occur -- and until a place in the natural polity of the country can be better filled by some modification of some one or more of its inhabitants.

"And such new places will depend on slow changes of climate -- or on the occasional immigration of new inhabitants -- and, probably, in a still more important degree, on some of the old inhabitants becoming slowly modified, with the new forms thus produced -- and the old ones acting and reacting on each other.

"Many forms, more especially amongst the classes which unite for each birth and wander much, may have separately been rendered sufficiently distinct to rank as representative species.

"In this, case, intermediate varieties between the several representative species and their common parent, must formerly have existed within each isolated portion of the land.

"These intermediate varieties will, from reasons already assigned -- namely from what we know of the actual distribution of closely allied or representative species, and likewise of acknowledged varieties -- exist in the intermediate zones in lesser numbers than the varieties which they tend to connect.

"From this cause alone the intermediate varieties will be liable to accidental extermination; and during the process of further modification through natural selection.

"It would be easy to show that there now exist carnivorous animals presenting close intermediate grades from strictly terrestrial to aquatic habits. As each exists by a struggle for life, it is clear that each must be well adapted to its place in nature.

"Look at the Mustela vision of North America, which has webbed feet, and which resembles an otter in its fur, short legs, and form of tail. During the summer this animal dives for and preys on fish, but during the long winter it leaves the frozen waters, and preys, like other pole-cats, on mice and land animals.

"Here we have the finest gradation from animals with their tails only slightly flattened -- and from others, as Sir J. Richardson has remarked, with the posterior part of their bodies rather wide and with the skin on their flanks rather full -- to the so-called flying squirrels. Flying squirrels have their limbs and even the base of the tail united by a broad expanse of skin, which serves as a parachute and allows them to glide through the air to an astonishing distance from tree to tree.

"We cannot doubt that each structure is of use to each kind of squirrel in its own country, by enabling it to escape birds or beasts of prey -- to collect food more quickly -- or, as there is reason to believe, to lessen the danger from occasional falls.

"But it does not follow from this fact that the structure of each squirrel is the best that it is possible to conceive under all possible conditions.

"Let the climate and vegetation change, let other competing rodents or new beasts of prey immigrate, or old ones become modified, and all analogy would lead us to believe that some at least of the squirrels would decrease in numbers or become exterminated, unless they also become modified and improved in structure in a corresponding manner.

"An extremely wide flank membrane stretches from the corners of the jaw to the tail, and includes the limbs with the elongated fingers. This flank-membrane is furnished with an extensor muscle.

"Although no graduated links of structure -- fitted for gliding through the air -- now connect the Galeopithecus with the other Insectivora, yet there is no difficulty in supposing that such links formerly existed -- and that each was developed in the same manner as with the less perfectly gliding squirrels -- each grade of structure having been useful to its possessor.

"Nor can I see any insuperable difficulty in further believing that the membrane connected fingers and fore-arm of the Galeopithecus might have been greatly lengthened by natural selection. This, as far as the organs of flight are concerned, would have converted the animal into a bat.

"Yet the structure of each of these birds is good for it, under the conditions of life to which it is exposed, for each has to live by a struggle.

"But it is not necessarily the best possible under all possible conditions. It must not be inferred from these remarks that any of the grades of wing-structure here alluded to -- which perhaps may all be the result of disuse -- indicate the steps by which birds actually acquired their perfect power of flight. But they serve to show what diversified means of transition are at least possible.

"Seeing that a few members of such water-breathing classes as the Crustacea and Mollusca are adapted to live on the land -- and seeing that we have flying birds and mammals -- flying insects of the most diversified types -- and formerly had flying reptiles, it is conceivable that flying-fish, which now glide far through the air, slightly rising and turning by the aid of their fluttering fins, might have been modified into perfectly winged animals.

"Furthermore, we may conclude that transitional states between structures fitted for very different habits of life will rarely have been developed at an early period in great numbers and under many subordinate forms.

"Thus, to return to our imaginary illustration of the flying-fish -- it does not seem probable that fishes capable of true flight would have been developed under many subordinate forms, for taking prey of many kinds in many ways, on the land and in the water -- until their organs of flight had come to a high stage of perfection, so as to have given them a decided advantage over other animals in the battle for life.

"In either case it would be easy for natural selection to adapt the structure of the animal to its changed habits, or exclusively to one of its several habits.

"It is, however, difficult to decide, and immaterial for us, whether habits generally change first and structure afterwards -- or whether slight modifications of structure lead to changed habits -- both probably often occurring almost simultaneously.

"Of cases of changed habits it will suffice merely to allude to that of the many British insects which now feed on exotic plants, or exclusively on artificial substances.

"Of diversified habits innumerable instances could be given. I have often watched a tyrant flycatcher (Saurophagus sulphuratus) in South America, hovering over one spot and then proceeding to another, like a kestrel -- and at other times standing stationary on the margin of water, and then dashing into it like a kingfisher at a fish.

"In our own country the larger titmouse (Parus major) may be seen climbing branches, almost like a creeper. It sometimes, like a shrike, kills small birds by blows on the head. I have many times seen and heard it hammering the seeds of the yew on a branch, and thus breaking them like a nuthatch.

"And such instances occur in nature.

"Can a more striking instance of adaptation be given than that of a woodpecker for climbing trees and seizing insects in the chinks of the bark? Yet in North America there are woodpeckers which feed largely on fruit, and others with elongated wings which chase insects on the wing.

"On the plains of La Plata, where hardly a tree grows, there is a woodpecker (Colaptes campestris) which has two toes before and two behind, a long pointed tongue, pointed tail-feathers, sufficiently stiff to support the bird in a vertical position on a post, but not so stiff as in the typical woodpeckers, and a straight strong beak.

"The beak, however, is not so straight or so strong as in the typical woodpeckers, but it is strong enough to bore into wood. Hence this Colaptes in all the essential parts of its structure is a woodpecker. Even in such trifling characters as the colouring, the harsh tone of the voice, and undulatory flight, its close blood-relationship to our common woodpecker is plainly declared.

"Yet, as I can assert, not only from my own observation, but from those of the accurate Azara, in certain large districts it does not climb trees, and it makes its nest in holes in banks!

"In certain other districts, however, this same woodpecker, as Mr. Hudson states, frequents trees, and bores holes in the trunk for its nest.

"What can be plainer than that the webbed feet of ducks and geese are formed for swimming? Yet there are upland geese with webbed feet which rarely go near the water.

"No one except Audubon has seen the frigate-bird, which has all its four toes webbed, alight on the surface of the ocean.

"On the other hand, grebes and coots are eminently aquatic, although their toes are only bordered by membrane.

"What seems plainer than that the long toes, not furnished with membrane, of the Grallatores are formed for walking over swamps and floating plants? The water-hen and landrail are members of this order, yet the first is nearly as aquatic as the coot, and the second nearly as terrestrial as the quail or partridge.

"In such cases, and many others could be given, habits have changed without a corresponding change of structure.

"But this seems to me only re-stating the fact in dignified language.

"He who believes in the struggle for existence and in the principle of natural selection, will acknowledge that every organic being is constantly endeavouring to increase in numbers. If any one being varies ever so little -- either in habits or structure, and thus gains an advantage over some other inhabitant of the same country -- it will seize on the place of that inhabitant, however different that may be from its own place.

"When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false. The old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science.

"Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist -- each grade being useful to its possessor, as is certainly the case -- if further, the eye ever varies and the variations be inherited, as is likewise certainly the case and if such variations should be useful to any animal under changing conditions of life -- then the difficulty of believing that a perfect and complex eye could be formed by natural selection -- though insuperable by our imagination -- should not be considered as subversive of the theory.