Russel Wallace : Alfred Russell Wallace (sic)
The mistake above alluded to has arisen from ignorance of the meaning of the word 'species,' the 'origin' of which Mr. Darwin undertakes to show. A species may be defined as a group of individuals of animals or plants which breed together freely and reproduce their like; whence it follows that all the individuals of a species, now living or which have lived, have descended from a few common ancestors, or perhaps from a single pair. Thus all horses, whether Shetland ponies, racers, or cart-horses, form one species, because they freely breed together, and are known to have all descended from a common stock.
By the same test the common ass, the kiang, the quagga, and the zebra, are each shown to be distinct species; for though sometimes two of these species will breed together, they do not do so freely, they do not reproduce their like but an intermediate form called a mule, and these mules are not capable of reproducing their kind, as are the offspring of any pairs of a single species. What Mr. Darwin did was to prove, by an overwhelming array of evidence and a connected chain of irresistible argument, that, just as all horses and all asses have each descended from a few common ancestors, so have all asses, horses, quaggas, and zebras descended from a much more remote common ancestral form; and that the same thing has occurred with every group of allied species. This is the 'origin of species' by descent with modification, or, in other words, by evolution; while 'natural selection' was the term applied to the set of natural causes [[p. 94]] which formed the motive power and guiding principle by which the change from one species to another was brought about.
In a very few years after the publication of this theory, it had literally extinguished among all thinking men the doctrine of special creation which had before largely prevailed; and some, who were its most violent opponents at the outset, now accept the fact of evolution as applied to almost every group of organised beings. At the present day there is perhaps no single naturalist of reputation who upholds that doctrine of the independent origin of each species of animal and plant, which was a very few years ago either tacitly accepted or openly maintained by the great majority of naturalists. Surely no such revolution in scientific thought was ever effected by one man in so short a period!
At first the opponents of Darwinism opposed evolution as well; but of late years the opposition is directed wholly to the adequacy of the causes which Mr. Darwin maintains are sufficient to explain the origin of each species from some pre-existing species, and therefore the origin of all existing species from some one or more ancestral forms. It is maintained that there are other laws at work besides natural selection, and Mr. Darwin has himself admitted that there probably are such. Most of the opponents of Darwinism argue in favour of some guiding or organising power, either internal or external, as absolutely necessary to the production of the kind and amount of variation necessary for the development of the various complex organs and special adaptations which characterise each important class of animals. Others go still further, and maintain that 'natural selection' is powerless to produce new species in any case, its function being to keep those which are produced in a state of health and perfection by weeding out all that are imperfect; or they argue that, so long as the 'cause of variation' is unknown, the power that preserves those variations when they have arisen plays a very subordinate part. These last writers maintain that the causes, whatever they are, which produce certain variations in certain species at certain times, are the true and only causes of the origin of species.
Now all these objections, in so far as they refer to the origin of the different species of one genus from a common ancestral species, or even of all the species and genera of one family from some still more remote ancestor, may, I think, be shown to be invalid; because we have direct evidence, almost amounting to demonstration, that changes to this extent are producible by the known laws of variation and the admitted action of natural selection. But when we go further back, and propose to account for the origin of distinct families, orders, and classes of animals by the same process, the evidence becomes far less clear and decisive. We find groups with organs of which no rudiment exists in other groups; we find classes differing radically in structure from other classes; and we have no direct [[p. 95]] evidence that changes of this nature are now in progress, as we have that the lesser changes resulting in new species and new genera are in progress.
Yet the evidence that these deeper and more important changes in the structure of organised beings have taken place by gradual steps through the ordinary processes of generation is overwhelming. The numerous intermediate links that have been discovered both among living and extinct animals, and especially the wonderful community perceptible in the embryological development of the most diverse living types, force upon us the conclusion that the entire animal and vegetable kingdoms owe the wonderfully diversified forms they now exhibit to one unbroken process of 'descent with modification' from a few primeval types. It is indeed generally assumed that if we go so far, we must admit one original type of living organism; but this does not seem necessary. By means of whatever laws we suppose living things first to have originated, why should not the primeval germs have appeared many times over, and in forms determined or modified by the infinitely varied chemical and physical conditions to be found in the crust of the earth? The identity of ultimate structure and wonderful similarities of development of all organisms may be due to the unity of the laws by which organic life was first produced; the diversity of the great types of animal and vegetable forms may be due to the operation of those laws at different places, acting on different combinations of elements, which are subject to unlike physical conditions.
The point here insisted upon is, that the origin of all organisms, living and extinct, by 'descent with modification,' is not necessarily the same thing, and is not included in, 'the origin of species by means of natural selection.' The latter we not only know has occurred, but we can follow the process step by step by means of known facts and known laws; the former, we are almost equally certain, has occurred, but we cannot trace its steps, and there may have been facts and laws involved of which we have no certain knowledge. The terms 'laws of growth,' 'laws of development,' 'laws of inheritance,' 'laws of variation,' 'laws of correlation,' 'direct action of the environment,' 'laws of habit and instinct,' with some others, are used to express the action of causes of which we are almost wholly ignorant, as we are of the nature of life itself. Now Mr. Darwin has himself admitted that there are these unknown causes at work, and that 'natural selection is the most important but not the exclusive means of modification.' There may be some question as to the term 'most important,' if, as is not improbable, the most radical differences in animals and their most important organs could not have been produced by it alone in the same way as the specific modifications of a genus or family may be produced. This, however, is a fair matter for discussion and research, and will probably continue to be so for many generations; and [[p. 96]] even if it should be ever proved that higher laws than 'natural selection' have brought about the more fundamental divergences of the animal and vegetable kingdoms, this will not be held to detract in any way from the greatness and the value of Mr. Darwin's work, any more than it will be held to detract from the greatness of Newton, if it should some day be demonstrated that the law of gravitation as expressed by him is not absolutely true, but that (as some physicists now suppose) it should be found to be subject to a higher law for remote stellar distances.
No thoughtful person can contemplate without amazement the phenomena presented by the development of animals. We see the most diverse forms--a mollusc, a frog, and a mammal--arising from apparently identical primitive cells, and progressing for a time by very similar initial changes, but thereafter each pursuing its highly complex and often circuitous course of development, with unerring certainty, by means of laws and forces of which we are totally ignorant. It is surely a not improbable supposition that the unknown power which determines and regulates this marvellous process may also determine the initiation of those more important changes of structure and those developments of new parts and organs which characterise the successive stages of the evolution of animal forms. In so far as Mr. Darwin denies the necessity of any such power, and maintains that the origin of all the diverse forms and types and all the complex structures of the organic world are due to identically the same laws and processes as are adequate to produce the different species of Rubus or of Canis, from some ancestral bramble or dog respectively, his opponents have undoubtedly a case well worthy of being argued out in the courts of science. They should, however, remember that no final judgment has been given or can be given while the evidence on both sides is not only circumstantial but imperfect and contradictory; and it would be well not to declare too confidently that Mr. Darwin's theory has hopelessly broken down, since a majority both of naturalists and geologists, whose evidence as experts will undoubtedly have great weight with the educated public, are at present altogether in his favour.
Leaving this great case to be discussed and argued in weighty volumes by specialists in science, I here propose to deal briefly with that much smaller but still important question, of the origin of the species of a genus or of a family--that is, of groups of organisms differing, as the wolf, dog, and fox among animals, or as the numerous species of oaks or of primulas among plants; and I hope to be able to show that in these cases there is hardly any room for doubt as to the mode in which the change from species to species has been effected.
We have to inquire, then, how it is that new species arise, supposing the world to have been then very much as it is now; and what [[p. 97]] becomes of them after they have arisen. In the first place we must remember that new species can only be formed when and where there is room for them. If a continent is well stocked with animals and plants, there is a balance between the different species, those best adapted to the varied existing conditions maintaining themselves in the largest numbers, while others, being only adapted to special conditions that occur in limited areas, are far less numerous; the former are common and widespread, the latter rare or local species. If the set of organisms in any country has existed for a sufficient time to have been subjected to all the varying conditions which occur during considerable cycles of climatal and other changes, the balance will have become well established, and so long as no change takes place in the conditions no new species will arise.
But now let us suppose some change to begin, either of climate or geography. The land may sink or it may be elevated, in the former case diminishing in area and perhaps becoming divided by an arm of the sea, in the latter case increasing in area and perhaps becoming united with extensive lands formerly separated from it; or the climate may become moister or drier, hotter or colder, more extreme or more equable, and any one of these changes or any combination of them would, it is easy to see, produce a special effect on the forms of life. The vegetation would in almost any case become changed, and this would affect both the insects and the higher animals in a variety of ways. New enemies or new competitors might be admitted, and these would certainly cause the extermination of some of the rarer species, and perhaps greatly reduce the numbers of those which had been most numerous. Others might, from the same general causes, obtain fresh supplies of food, or have opened to them fresh areas over which to spread themselves.
These are the first and most obvious effects of such changes, but there are others still more important, and not less certain to be produced. We have supposed each of the species which inhabited the country to be well adapted to the conditions of its existence, to be able to obtain food for itself and young, to protect itself against all kinds of enemies, and to be able to resist the ordinary inclemencies of the seasons, and to do all this in competition with the numerous other species by which it was surrounded. But now all these conditions and surroundings are undergoing change, and, in order to become equally well adapted to the new conditions, some of the species will require to undergo a corresponding change, either in structure, habits, colour, or some other characters. New enemies may necessitate greater swiftness, or greater cunning, or less conspicuous colours; less abundant food may necessitate some modification in structure better adapted to secure it, or the means of ranging over a wider area to search for it; while a severer climate may necessitate a thicker covering, or more nourishing food, or new kinds of shelter. To [[p. 98]] bring about these changes, 'variation' and the 'struggle for existence' come into play. Each year the old and less adapted forms die out, while those variations which are more in harmony with the new conditions constantly survive; and this process, continued for many hundreds or thousands of successive generations, at length results in the production of one or more new species.
We now come to the difficulty which has been repeatedly put forward, and which seems very great to all who have not studied groups of species as they occur in nature, and which is expressed in the question 'How comes it that variations of the right kind and sufficient in amount have always occurred just when they were wanted, so as to form the endless series of new species that have arisen?' and it is more especially to answer this question that the present paper has been written.
Few persons consider how largely and universally all animals are varying; yet it is certain that if we could examine all the individuals of any common species, we should find considerable differences, not only in size and colour but in the form and proportions of all the parts and organs of the body. In our domesticated animals we know this to be the case, and it is by means of the continued selection of such slight varieties to breed from that all our extremely varied domestic animals and cultivated plants have been produced. Think of the difference in every limb, in every bone and muscle, and probably in every part, internal and external, of the whole body, between a greyhound and a bull-dog! Yet, if we had the whole series of ancestors of these two breeds before us, we should find them gradually converge till they reached the same original type, while between no two successive generations would there be any greater difference than now sometimes occurs in the same litter. It is often thought, however, that wild animals do not vary sufficiently to enable any such change as this to be brought about in the same limited time; and though naturalists are well aware that there is little, if any, difference in this respect between wild and domesticated species, it is only recently that they have been able to adduce positive proof that this is the case.
We owe this proof to an American naturalist, Mr. J. A. Allen,1 who has made an elaborate series of observations and measurements of the mammals, and more especially of the birds, of the United States; and he finds a wonderful and altogether unsuspected amount of variation between individuals of the same species even when inhabiting the same locality. They differ in the general tint, and in the distribution of the colours and markings; in general size, and [[p. 99]] in proportions; in the length of the head, feet, wings, and tail; in the length of particular feathers, thus altering the shape of the wing or tail; in the length of the tarsi and of the separate toes; and in the length, width, thickness, and curvature of the bill. These variations are by no means small in amount or requiring very accurate measurements for their detection, since they often reach one-seventh, one-sixth, or sometimes even one-fourth, of the entire average dimensions. Thus, in twelve species of small birds, all taken in the same locality, the variation in twenty-five or thirty specimens of the same sex and age was, in the length of the folded wing, from 14.5 to 21 per cent., and in the length of the tail from 14 to 23.4 per cent. If we take individual cases, we find equally striking facts. Wilson's thrush (Turdus fuscescens) was found to vary in length of wing from 3.58 to 4.15 inches, and in the tail from 3.55 to 4 inches. In the Blue-bird (Sialia sialis) the middle toe varied from 0.77 to 0.91 inch, and the hind toe from 0.58 to 0.72 inch; while the bill varied from 0.45 to 0.56 inch in length, and from 0.30 to 0.38 inch in width. In the Yellow-crowned Warbler (Dendrca coronata) the quills vary in proportionate length, so that the first, the second, the third, or the fourth, is sometimes the longest; and a similar variation of the wing, involving a change of proportion between two or more of the feathers, is recorded in eleven species of birds. Colour and marking vary to a still greater extent. The dark streaks on the under parts of the American Song-sparrow (Melospiza melodia) are sometimes reduced to narrow lines, while in other specimens they are so enlarged as to cover the greater part of the breast and sides of the body, sometimes uniting on the middle of the breast into a nearly continuous patch. In the small spotted Wood-thrushes (of the sub-genus Hylocichla) not only does the general tint of different parts vary greatly, but this is accompanied by great variation in the markings, some specimens being very pale with indistinct narrow lines on the breast, while others have dark plumage and dark, broad, triangular markings. It must be remembered that all these differences are independent of those due to age, sex, season, or locality, and consist solely of what may be termed the normal individual variation of the species.
It is, however, often supposed that variations occur at any one time in single characters only, all the rest remaining invariable; and it is objected that to adapt a creature to new conditions it must be modified in several ways at once. But a reference to the tables given by Mr. Allen shows that this coincident variation of several characters does exist to a remarkable extent. He has given the variation of no less than nineteen characters in ten species of birds, from a comparison in each case of only twenty specimens, all of the same sex, all fully adult, and all taken in the same localities. On marking the specimens which have each character at a maximum [[p. 100]] or minimum development, we find the most curious combinations. We find, for example, that the largest specimens have not always the longest wings or tails, or the smallest specimens the shortest; the proportion of the different parts of the wing varies quite regardless of the actual dimensions; the length of any toe varies independently of the length of the tarsus; a long head sometimes goes with a short, sometimes with a long, wing; while the width of the bill seems to vary independently of its length or of any of the other parts of the body. All these variations, too, are very considerable in amount. Thus among twenty male Baltimore Orioles the total length varied from 7 to 8 inches; the wing from 3.45 to 3.85 inches; the tail from 2.70 to 3.10 inches; the primaries extended beyond the secondaries from 0.56 to 0.90 inch; the tail extended beyond the upper coverts from 1.37 to 1.87 inches; the tarsus varied from 0.83 to 1.02 inch; the hind toe varied from 0.62 to 0.75 inch, and the middle toe from 0.82 to 1.00 inch; the head varied in length from 1.50 to 1.62 inches; the beak in length from 0.74 to 0.84 inch, and in width from 0.32 to 0.38 inch. And if these differences and these combinations, indicating many diverging proportions between two or more characters, are found among only twenty specimens, we may certainly expect much greater differences in every character, and these differences combined in an endless variety of ways, among the millions of individuals which constitute every common species. Not only, therefore, is it clear that there is, among birds at all events, ample individual variation for natural selection to work upon, but, what is even more important, that coincident variations in every conceivable combination are also available.
Among mammalia we have fewer materials for comparison, but there is good reason to believe that they are quite as variable as birds, if not even more so. Among twenty males of the Grey Squirrel, whose dimensions are given by Mr. Allen, we find the length of the tail to vary as 3 to 4, of the fore foot as 9 to 11, and of the hind foot as 6 to 7. The Virginian Opossum also varies greatly in colour, and in the size and proportions of all the parts, including the skull, the variation amounting to nearly twenty per cent.
If now we consider the population of a species with regard to any particular character or combination of characters, we may divide it into three groups--a central group in which the mean or average development prevails with little variation, one in which the character is excessively, and one in which it is little developed. These groups would not be equal extent, the central portion--that in which the mean characteristics prevailed--being, in accordance with the law of averages, much more numerous than the extremes; perhaps twice or even three times as great as either of them, and forming such a series as the following:--Maximum development 10, mean 30, minimum 10. These figures, whatever their exact [[p. 101]] proportions, would probably be pretty constant, for we have no reason to believe that the mean characters, or the amount of variation of a species, change materially from year to year or from century to century; and we may therefore look upon the central and most numerous group as presenting the typical form of the species, being that which is best adapted to the conditions in which it has actually to exist, while the extremes, being less perfectly adapted, are continually weeded out by natural selection.
Besides the individual variation above noticed in birds of the same locality, another set of variations appears in birds of the same species inhabiting different localities. In North America birds decrease in size as they inhabit localities further south, while they become larger as we go north. In mammalia, on the other hand, there is generally a decrease of size both north and south from a central position where the species is at a maximum. Strange to say, the bill of most birds increases in length towards the south, sometimes relatively, but in other cases absolutely, so that the smaller southern birds sometimes have a bill actually longer than the northern larger individuals. This peculiarity occurs in the genera Quiscalus, Agelæus, Troglodytes, Seiurus, &c., and is illustrated by numerous figures in Mr. Allen's work. In some cases, as in the American crow, the bill is so much larger in the south that the Florida birds have been recognised as a distinct named variety.
Colour also varies greatly in correspondence to latitude and longitude. Dark-coloured birds are said to become blacker towards the south; in others the yellow or red bands become deeper; while in those transversely banded, the dark bands become broader, and the light ones narrower. Those with white spots or bands have them smaller in the south, and sometimes lose them altogether. These differences are sometimes so great that the extreme northern and southern forms might be considered distinct species were it not for the perfect gradation of intermediate types in the intervening localities. There is also an increase of intensity of colour from east to west, as exhibited by the same or by closely allied representative species inhabiting the Atlantic and Pacific coasts respectively. In the desert plains of the interior, however, the colours are paler than on either coast; but this is no doubt a protective modification, assimilating the tints of animals to the rock or surface soil on which they dwell. In some cases well-marked varieties of the same species appear to be confined to the Eastern States and to California respectively, as, for example, the eastern and western forms of Bewick's Wren (Thryothorus Bewickii), which differ greatly in the length of the bill, although otherwise almost identical; and as these two forms do not, so far as yet known, anywhere intermingle, they afford a good example of the first step in the formation of a new species. The beautiful purple finch (Carpodacus purpureus) of the Eastern States, and its [[p. 102]] western form which has been named californicus, perhaps form another example; but until the range of these birds is fully and accurately determined we cannot be sure that there is not some limited area where the two forms intermingle and their distinctive characters disappear.
From the fact of variation, so extensive as regards the number of variable characters and so large in absolute amount as has now been proved to exist in many species, we may fairly draw the conclusion that analogous variation, sometimes of less and sometimes of greater extent, is a general characteristic of animals in a state of nature; and with such materials to work with it becomes easy to understand how new species may arise. For example, the peculiar physical or organic conditions that render one part of the area occupied by a species better adapted to an extreme variety may become intensified. The most extreme variations in this direction will then have the advantage, and will multiply at the expense of the rest. If this change of condition should extend over the whole area occupied by the species, this one extreme form will replace all the others; while, if the area should be cut in two by subsidence or elevation, the conditions of the two portions may be modified in opposite directions, each becoming adapted to one extreme form. The original type of the species will then have become extinct, being replaced by two species, each distinguished by a combination of certain extreme characters which had before existed in some of its varieties.
The changes of conditions which lead to such selection of varieties are very diverse in their nature; and new species may thus be formed diverging in many ways from the parent stock. The climate may change from moist to dry, or the reverse, or the temperature may increase or diminish during long periods, in either case requiring some corresponding change of constitution, of covering, of vegetable or of insect food--to be met by the selection of variations of colour or of swiftness, of length of bill, or of strength of claws. Again, competitors or enemies may arrive from other countries, giving the advantage to such varieties as can change their food, or by swifter flight or greater wariness can escape their new foes. In this way several series of changes may occur, each brought about by the pressure of changed conditions; and thus what was before a single species may become transformed into a group of allied species, differing from each other in a number of slight characters, just as we find them in nature.
Let us now see how the same principles will explain the origin of genera. A genus is a group of allied species which differs from all other groups in some well-marked characters of a structural rather than of a superficial nature. For example, species of the same genus usually differ from each other in size, in colour or markings, in the proportion of the limbs or other organs, and in the form and size of such superficial appendages as horns, crests, manes, and the [[p. 103]] accessory ornamental plumes of birds; but they generally agree in the form and structure of important organs, as the teeth, the bill, the feet, and the wings. But when two groups of species differ from each other constantly, and to a well-marked degree, in one or more of these latter characters, they are said to belong to distinct genera; and we have seen that species vary in these as well as the more superficial characters--the bill, the feet, and the wings varying in size and proportions just as frequently as do the colours or the ornamental parts of the plumage. If then, in any portion of the area occupied by a species, some important change of habits becomes useful to it, all such structural variations as facilitate the change will be accumulated by natural selection, and when they have thus acquired the proportions most beneficial under the altered conditions, we shall have the first species of a new genus.
A creature which has been thus modified in important characters will form a new type, specially adapted to fill its place in the economy of nature. It will almost certainly have arisen from an extensive or dominant group, because such only are sufficiently rich in individuals to afford an ample supply of the needful variations, and it will therefore inherit the vigour of constitution and adaptability to a wide range of conditions which gave success to its ancestors. It will thus have every chance in its favour in the struggle for existence; it may spread widely and displace some of its nearest allies, and in doing this will extend into new areas, where it will be subject to a somewhat altered set of conditions, and by further variation and selection may become the parent of a number of subordinate species. It will now have become a dominant genus, occupying an entire continent, or perhaps even two or more continents, spreading on all sides till it meets with competing forms better adapted to the conditions which there prevail.
Such a genus may continue to exist during long geological epochs. Bats of the genus Vespertilio lived in the Eocene period, and still range over all the globe, while fossil land shells of the genus Pupa, hardly distinguishable from some now living, are found in the ancient carboniferous deposits. Generally, however, a time comes to every genus when either physical changes, or competing forms, or new enemies, are too much for it, and it begins to lose its supremacy. First one, then another of its species dwindle away and become extinct, till at last two or three only remain. Sometimes these soon follow the others, and the whole genus dies out, as thousands of genera have died out during the long course of the earth's life-history; but it also often happens that a few species continue to maintain themselves in areas where they are removed from the influences that have exterminated their fellows. Thus the mudfish of Queensland (Ceratodus Forsteri) and the Trigonia of the Australian seas are the only living representatives of genera which lived in the Triassic period.
[[p. 104]] I have now, I think, shown that one of the most general objections to natural selection as producing new species--namely, that there are enormous chances against the right kind of variations occurring just when they are required--is utterly fallacious, by proving that there is ample variation of every kind constantly occurring among animals in a state of nature. It has also been shown that many different kinds of variation are occurring at the same time, and in endlessly varied combinations, so that any required combination of characters could be selected as well as any single character. And when we consider the extreme slowness of almost all the changes of conditions which lead to the selection of new forms, and the enormous selecting power brought to bear owing to the rapid increase and corresponding great annual mortality among all animals, it is impossible to doubt that the means are adequate to the result. To bring these means clearly before our readers, let us suppose that a pair of birds produce every year six young, and that they live for five years. We thus have thirty birds out of which to replace the two, so that, on the average, at least twenty-eight must die during this time, and many more if any of these live to breed along with their parents. This gives us, as a minimum, a destruction every five years of fourteen times as many birds as exist at any one time. Now let us suppose a change going on which renders it beneficial for a species to obtain longer wings in order to escape from some enemy, and a stronger bill to enable it to capture some fresh insect, both of which (the enemy and the insect) are gradually increasing in the country. Variations of both these kinds occur in abundance every year, to an amount measured by ten or twenty per cent. of the average dimensions. Either of the variations would be useful and would be preserved separately, while the combined variation would be doubly useful and would also be preserved whenever it appeared. A race in which these two characters were from ten to twenty per cent. above the average would therefore be easily produced in twenty or fifty years; while in a thousand or five thousand years a change amounting to thirty or forty per cent.--far greater than distinguishes many species--would probably be brought about. This illustration, I think, renders it clear that the extreme slowness of the action of natural selection, on which Mr. Darwin repeatedly dwells, is by no means an essential characteristic of it, but is only due to the fact that physical and other conditions usually change with extreme slowness. But if, as must often have happened, conditions have changed with comparative rapidity, then the enormous amount of individual variation, which would be taken advantage of every year by the survival of the fittest, might effect changes in a single century quite as great as those which distinguish nearly allied species.
Another objection which is dwelt upon with constant reiteration by Mr. Darwin's critics is, that he has not shown the cause of [[p. 105]] variation, and that whatever it is that causes variation, that is the real 'origin of species.' This has always seemed to me one of the most unmeaning and irrational of objections, because every explanation must take as a basis well-known facts to explain obscure phenomena. When the geologist explains how the contour of a country has been formed by rain and ice, it is not said that he has explained nothing unless he goes on to show exactly how rain and snow are formed, or even goes further back to the cause of gravitation which is really what gives them all their power to do any work; and when the physicist explains how thunder and lightning are produced by a reference to the electric spark and its accompanying sound, he is not told that the explanation is valueless till he has discovered the nature and cause of electricity itself.
But we may, I think, go further, and say that variation is an ultimate fact of nature, and needs no other explanation than a reference to general principles which indicate that it cannot fail to exist. Does any one ask for a reason why no two gravel-stones or beach-pebbles, or even grains of sand, are absolutely identical in size, shape, surface, colour, and composition? When we trace back the complex series of causes and forces that have led to the production of these objects, do we not see that their absolute identity would be more remarkable than their diversity? So, when we consider how infinitely more complex have been the forces that have produced each individual animal or plant, and when we know that no two animals can possibly have been subject to identical conditions throughout the entire course of their development, we see that perfect identity in the result would be opposed to everything we know of natural agencies. But variation is merely the absence of identity, and therefore requires no further explanation; neither do the diverse amounts of variation, for they correspond to the countless diversities of conditions to which animals have been exposed either during their own development or that of their ancestors.
This objection has really its only possible justification in the ignorant belief that variations of any tangible amount are rare events occurring at long intervals; and therefore that when any combination of special variations was needed to bring an animal into harmony with changed conditions, the number of individuals varying would not be sufficiently great to prevent their being completely swamped by the typical unvarying forms. Had such been the case, some agency capable of producing a considerable amount of variation when required would undoubtedly have been needed, and this unknown agency might fairly have been claimed to be one of the most important factors in the 'origin of species.' But now that it is proved by a series of careful observations, that a large percentage of the individuals of most species vary, in each successive generation, to an amount far greater than is required for natural selection to act [[p. 106]] upon, the whole difficulty ceases to exist. Variation is seen to be one of the most constant and universal facts of nature, always producing what may be termed the raw materials of species in overflowing abundance, so that, whenever and wherever alteration of the conditions of existence is going on, there is always ready to hand an ample stock of varying organisms, by means of which an almost exact adjustment to those conditions may be kept up.
The facts and arguments now adduced will, it is hoped, enable intelligent readers who are not naturalists to form a clear conception of what is really meant by 'the origin of species by means of natural selection,' and will satisfy them that the most common and what seem at first sight to be the most weighty objections to it, owe all their force to the ignoring of some of the best established facts in natural history.
I have also attempted to show that the causes which have produced the separate species of one genus, of one family, or perhaps of one order from a common ancestor, are not necessarily the same as those which have produced the separate orders, classes, and sub-kingdoms from more remote common ancestors. That all have been alike produced by 'descent with modification' from a few primitive types the whole body of evidence clearly indicates; but while individual variation with natural selection is proved to be adequate for the production of the former, we have no proof and hardly any evidence that it is adequate to initiate those important divergences of type which characterise the latter.
1. 'On the Mammals and Winter Birds of East Florida; with an Examination of certain assumed Specific Characters in Birds, and a Sketch of the Bird Faunæ of Eastern North America.' By J. A. Allen. (Bulletin of the Museum of Comparative Zoology at Harvard College, Cambridge, Mass., vol. ii. No. 3.) [[on p. 98]]