Exact measurements of large series of comparable skulls of the mammalia are not easily found, but from those available I have prepared three diagrams (Figs. 14, 15, and 16), in order to exhibit the facts of variation in this very important organ. The first shows the variation in ten specimens of the common wolf (Canis lupus) from one district in North America, and we see that it is not only large in amount, but that each part exhibits a considerable independent variability.[23]
In Diagram 15 we have the variations of eight skulls of the Indian Honey-bear (Ursus l.a.b.i.atus), as tabulated by the late Dr. J.E. Gray of the British Museum. For such a small number of specimens the amount of variation is very large--from one-eighth to one-fifth of the mean size,--while there are an extraordinary number of instances of independent variability. In Diagram 16 we have the length and width of twelve skulls of adult males of the Indian wild boar (Sus cristatus), also given by Dr. Gray, exhibiting in both sets of measurements a variation of more than one-sixth, combined with a very considerable amount of independent variability.[24]
[Ill.u.s.tration: FIG. 15.--Variation of 8 skulls (Ursus l.a.b.i.atus).]
[Ill.u.s.tration: FIG. 16.]
The few facts now given, as to variations of the internal parts of animals, might be multiplied indefinitely by a search through the voluminous writings of comparative anatomists. But the evidence already adduced, taken in conjunction with the much fuller evidence of variation in all external organs, leads us to the conclusion that wherever variations are looked for among a considerable number of individuals of the more common species they are sure to be found; that they are everywhere of considerable amount, often reaching 20 per cent of the size of the part implicated; and that they are to a great extent independent of each other, and thus afford almost any combination of variations that may be needed.
It must be particularly noticed that the whole series of variation-diagrams here given (except the three which ill.u.s.trate the number of varying individuals) in every case represent the actual amount of the variation, not on any reduced or enlarged scale, but as it were life-size. Whatever number of inches or decimals of an inch the species varies in any of its parts is marked on the diagrams, so that with the help of an ordinary divided rule or a pair of compa.s.ses the variation of the different parts can be ascertained and compared just as if the specimens themselves were before the reader, but with much greater ease.
In my lectures on the Darwinian theory in America and in this country I used diagrams constructed on a different plan, equally ill.u.s.trating the large amount of independent variability, but less simple and less intelligible. The present method is a modification of that used by Mr.
Francis Galton in his researches on the theory of variability, the upper line (showing the variability of the body) in Diagrams 4, 5, 6, and 13, being laid down on the method he has used in his experiments with sweet-peas and in pedigree moth-breeding.[25] I believe, after much consideration, and many tedious experiments in diagram-making, that no better method can be adopted for bringing before the eye, both the amount and the peculiar features of individual variability.
_Variations of the Habits of Animals._
Closely connected with those variations of internal and external structure which have been already described, are the changes of habits which often occur in certain individuals or in whole species, since these must necessarily depend upon some corresponding change in the brain or in other parts of the organism; and as these changes are of great importance in relation to the theory of instinct, a few examples of them will be now adduced.
The Kea (Nestor notabilis) is a curious parrot inhabiting the mountain ranges of the Middle Island of New Zealand. It belongs to the family of Brush-tongued parrots, and naturally feeds on the honey of flowers and the insects which frequent them, together with such fruits or berries as are found in the region. Till quite recently this comprised its whole diet, but since the country it inhabits has become occupied by Europeans it has developed a taste for a carnivorous diet, with alarming results.
It began by picking the sheepskins hung out to dry or the meat in process of being cured. About 1868 it was first observed to attack living sheep, which had frequently been found with raw and bleeding wounds on their backs. Since then it is stated that the bird actually burrows into the living sheep, eating its way down to the kidneys, which form its special delicacy. As a natural consequence, the bird is being destroyed as rapidly as possible, and one of the rare and curious members of the New Zealand fauna will no doubt shortly cease to exist.
The case affords a remarkable instance of how the climbing feet and powerful hooked beak developed for one set of purposes can be applied to another altogether different purpose, and it also shows how little real stability there may be in what appear to us the most fixed habits of life. A somewhat similar change of diet has been recorded by the Duke of Argyll, in which a goose, reared by a golden eagle, was taught by its foster-parent to eat flesh, which it continued to do regularly and apparently with great relish.[26]
Change of habits appears to be often a result of imitation, of which Mr.
Tegetmeier gives some good examples. He states that if pigeons are reared exclusively with small grain, as wheat or barley, they will starve before eating beans. But when they are thus starving, if a bean-eating pigeon is put among them, they follow its example, and thereafter adopt the habit. So fowls sometimes refuse to eat maize, but on seeing others eat it, they do the same and become excessively fond of it. Many persons have found that their yellow crocuses were eaten by sparrows, while the blue, purple, and white coloured varieties were left untouched; but Mr. Tegetmeier, who grows only these latter colours, found that after two years the sparrows began to attack them, and thereafter destroyed them quite as readily as the yellow ones; and he believes it was merely because some bolder sparrow than the rest set the example. On this subject Mr. Charles C. Abbott well remarks: "In studying the habits of our American birds--and I suppose it is true of birds everywhere--it must at all times be remembered that there is less stability in the habits of birds than is usually supposed; and no account of the habits of any one species will exactly detail the various features of its habits as they really are, in every portion of the territory it inhabits."[27]
Mr. Charles Dixon has recorded a remarkable change in the mode of nest-building of some common chaffinches which were taken to New Zealand and turned out there. He says: "The cup of the nest is small, loosely put together, apparently lined with feathers, and the walls of the structure are prolonged for about 18 inches, and hang loosely down the side of the supporting branch. The whole structure bears some resemblance to the nests of the hangnests (Icteridae), with the exception that the cavity is at the top. Clearly these New Zealand chaffinches were at a loss for a design when fabricating their nest.
They had no standard to work by, no nests of their own kind to copy, no older birds to give them any instruction, and the result is the abnormal structure I have just described."[28]
These few examples are sufficient to show that both the habits and instincts of animals are subject to variation; and had we a sufficient number of detailed observations we should probably find that these variations were as numerous, as diverse in character, as large in amount, and as independent of each other as those which we have seen to characterise their bodily structure.
_The Variability of Plants._
The variability of plants is notorious, being proved not only by the endless variations which occur whenever a species is largely grown by horticulturists, but also by the great difficulty that is felt by botanists in determining the limits of species in many large genera. As examples we may take the roses, the brambles, and the willows as well ill.u.s.trating this fact. In Mr. Baker"s _Revision of the British Roses_ (published by the Linnean Society in 1863), he includes under the single species, Rosa canina--the common dog-rose--no less than twenty-eight named _varieties_ distinguished by more or less constant characters and often confined to special localities, and to these are referred about seventy of the _species_ of British and continental botanists. Of the genus Rubus or bramble, _five_ British species are given in Bentham"s _Handbook of the British Flora_, while in the fifth edition of Babington"s _Manual of British Botany_, published about the same time, no less than _forty-five_ species are described. Of willows (Salix) the same two works enumerate _fifteen_ and _thirty-one_ species respectively. The hawkweeds (Hieracium) are equally puzzling, for while Mr. Bentham admits only seven British species, Professor Babington describes no less than thirty-two, besides several named varieties.
A French botanist, Mons. A. Jordan, has collected numerous forms of a common little plant, the spring whitlow-gra.s.s (Draba verna); he has cultivated these for several successive years, and declares that they preserve their peculiarities unchanged; he also says that they each come true from seed, and thus possess all the characteristics of true species. He has described no less than fifty-two such species or permanent varieties, all found in the south of France; and he urges botanists to follow his example in collecting, describing, and cultivating all such varieties as may occur in their respective districts. Now, as the plant is very common almost all over Europe and ranges from North America to the Himalayas, the number of similar forms over this wide area would probably have to be reckoned by hundreds if not by thousands.
The cla.s.s of facts now adduced must certainly be held to prove that in many large genera and in some single species there is a very large amount of variation, which renders it quite impossible for experts to agree upon the limits of species. We will now adduce a few striking cases of individual variation.
The distinguished botanist, Alp. de Candolle, made a special study of the oaks of the whole world, and has stated some remarkable facts as to their variability. He declares that on the same branch of oak he has noted the following variations: (1) In the length of the petiole, as one to three; (2) in the form of the leaf, being either elliptical or obovoid; (3) in the margin being entire, or notched, or even pinnatifid; (4) in the extremity being acute or blunt; (5) in the base being sharp, blunt, or cordate; (6) in the surface being p.u.b.escent or smooth; (7) the perianth varies in depth and lobing; (8) the stamens vary in number, independently; (9) the anthers are mucronate or blunt; (10) the fruit stalks vary greatly in length, often as one to three; (11) the number of fruits varies; (12) the form of the base of the cup varies; (13) the scales of the cup vary in form; (14) the proportions of the acorns vary; (15) the times of the acorns ripening and falling vary.
Besides this, many species exhibit well-marked varieties which have been described and named, and these are most numerous in the best-known species. Our British oak (Quercus robur) has twenty-eight varieties; Quercus Lusitanica has eleven; Quercus calliprinos has ten; and Quercus coccifera eight.
A most remarkable case of variation in the parts of a common flower has been given by Dr. Hermann Muller. He examined two hundred flowers of Myosurus minimus, among which he found _thirty-five_ different proportions of the sepals, petals, and anthers, the first varying from four to seven, the second from two to five, and the third from two to ten. Five sepals occurred in one hundred and eighty-nine out of the two hundred, but of these one hundred and five had three petals, forty-six had four petals, and twenty-six had five petals; but in each of these sets the anthers varied in number from three to eight, or from two to nine. We have here an example of the same amount of "independent variability" that, as we have seen, occurs in the various dimensions of birds and mammals; and it may be taken as an ill.u.s.tration of the kind and degree of variability that may be expected to occur among small and little specialised flowers.[29]
In the common wind-flower (Anemone nemorosa) an almost equal amount of variation occurs; and I have myself gathered in one locality flowers varying from 7/8 inch to 1-3/4 inch in diameter; the bracts varying from 1-1/2 inch to 4 inches across; and the petaloid sepals either broad or narrow, and varying in number from five to ten. Though generally pure white on their upper surface, some specimens are a full pink, while others have a decided bluish tinge.
Mr. Darwin states that he carefully examined a large number of plants of Geranium phaeum and G. pyrenaic.u.m (not perhaps truly British but frequently found wild), which had escaped from cultivation, and had spread by seed in an open plantation; and he declares that "the seedlings varied in almost every single character, both in their flowers and foliage, to a degree which I have never seen exceeded; yet they could not have been exposed to any great change of their conditions."[30]
The following examples of variation in important parts of plants were collected by Mr. Darwin and have been copied from his unpublished MSS.:--
"De Candolle (_Mem. Soc. Phys. de Geneve_, tom. ii. part ii. p. 217) states that Papaver bracteatum and P. orientale present indifferently two sepals and four petals, or three sepals and six petals, which is sufficiently rare with other species of the genus."
"In the Primulacae and in the great cla.s.s to which this family belongs the unilocular ovarium is free, but M. Dubury (_Mem. Soc. Phys. de Geneve_, tom. ii. p. 406) has often found individuals in Cyclamen hederaefolium, in which the base of the ovary was connected for a third part of its length with the inferior part of the calyx."
"M. Aug. St. Hilaire (Sur la Gyn.o.base, _Mem. des Mus. d"Hist. Nat._, tom. x. p. 134), speaking of some bushes of the Gomphia oleaefolia, which he at first thought formed a quite distinct species, says: "Voila donc dans un meme individu des loges et un style qui se rattachent tantot a un axe vertical, et tantot a un gyn.o.base; donc celui-ci n"est qu"un axe veritable; mais cet axe est deprime au lieu d"etre vertical."
He adds (p. 151), "Does not all this indicate that nature has tried, in a manner, in the family of Rutaceae to produce from a single multilocular ovary, one-styled and symmetrical, several unilocular ovaries, each with its own style." And he subsequently shows that, in Xanthoxylum monogynum, "it often happens that on the same plant, on the same panicle, we find flowers with one or with two ovaries;" and that this is an important character is shown by the Rutaceae (to which Xanthoxylum belongs), being placed in a group of natural orders characterised by having a solitary ovary."
"De Candolle has divided the Cruciferae into five sub-orders in accordance with the position of the radicle and cotyledons, yet Mons. T.
Gay (_Ann. des Scien. Nat._, ser. i. tom. vii. p. 389) found in sixteen seeds of Petrocallis Pyrenaica the form of the embryo so uncertain that he could not tell whether it ought to be placed in the sub-orders "Pleurorhizee" or "Notor-hizee"; so again (p. 400) in Cochlearia saxatilis M. Gay examined twenty-nine embryos, and of these sixteen were vigorously "pleurorhizees," nine had characters intermediate between pleuro-and notor-hizees, and four were pure notor-hizees."
"M. Raspail a.s.serts (_Ann. des Scien. Nat._, ser. i. tom. v. p. 440) that a gra.s.s (Nostus Borbonicus) is so eminently variable in its floral organisation, that the varieties might serve to make a family with sufficiently numerous genera and tribes--a remark which shows that important organs must be here variable."
_Species which vary little._
The preceding statements, as to the great amount of variation occurring in animals and plants, do not prove that all species vary to the same extent, or even vary at all, but, merely, that a considerable number of species in every cla.s.s, order, and family do so vary. It will have been observed that the examples of great variability have all been taken from common species, or species which have a wide range and are abundant in individuals. Now Mr. Darwin concludes, from an elaborate examination of the floras and faunas of several distinct regions, that common, wide ranging species, as a rule, vary most, while those that are confined to special districts and are therefore comparatively limited in number of individuals vary least. By a similar comparison it is shown that species of large genera vary more than species of small genera. These facts explain, to some extent, why the opinion has been so prevalent that variation is very limited in amount and exceptional in character. For naturalists of the old school, and all mere collectors, were interested in species in proportion to their rarity, and would often have in their collections a larger number of specimens of a rare species than of a species that was very common. Now as these rare species do really vary much less than the common species, and in many cases hardly vary at all, it was very natural that a belief in the fixity of species should prevail. It is not, however, as we shall see presently, the rare, but the common and widespread species which become the parents of new forms, and thus the non-variability of any number of rare or local species offers no difficulty whatever in the way of the theory of evolution.
_Concluding Remarks._
We have now shown in some detail, at the risk of being tedious, that individual variability is a general character of all common and widespread species of animals or plants; and, further, that this variability extends, so far as we know, to every part and organ, whether external or internal, as well as to every mental faculty. Yet more important is the fact that each part or organ varies to a considerable extent independently of other parts. Again, we have shown, by abundant evidence, that the variation that occurs is very large in amount--usually reaching 10 or 20, and sometimes even 25 per cent of the average size of the varying part; while not one or two only, but from 5 to 10 per cent of the specimens examined exhibit nearly as large an amount of variation. These facts have been brought clearly before the reader by means of numerous diagrams, drawn to scale and exhibiting the actual variations in inches, so that there can be no possibility of denying either their generality or their amount. The importance of this full exposition of the subject will be seen in future chapters, when we shall frequently have to refer to the facts here set forth, especially when we deal with the various theories of recent writers and the criticisms that have been made of the Darwinian theory.
A full exposition of the facts of variation among wild animals and plants is the more necessary, because comparatively few of them were published in Mr. Darwin"s works, while the more important have only been made known since the last edition of _The Origin of Species_ was prepared; and it is clear that Mr. Darwin himself did not fully recognise the enormous amount of variability that actually exists. This is indicated by his frequent reference to the extreme slowness of the changes for which variation furnishes the materials, and also by his use of such expressions as the following: "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"
(_Origin_, p. 66). And again, after speaking of changed conditions "affording a better chance of the occurrence of favourable variations,"
he adds: "_Unless such occur natural selection can do nothing_"
(_Origin_, p. 64). These expressions are hardly consistent with the fact of the constant and large amount of variation, of every part, in all directions, which evidently occurs in each generation of all the more abundant species, and which must afford an ample supply of favourable variations whenever required; and they have been seized upon and exaggerated by some writers as proofs of the extreme difficulties in the way of the theory. It is to show that such difficulties do not exist, and in the full conviction that an adequate knowledge of the facts of variation affords the only sure foundation for the Darwinian theory of the origin of species, that this chapter has been written.
FOOTNOTES:
[Footnote 16: _Foraminifera_, preface, p. x.]
[Footnote 17: _United States Geological Survey of the Territories_, 1874.]
[Footnote 18: _Proceedings of the Entomological Society of London_, 1875, p. vii.]
[Footnote 19: _Ann. des Sci. Nat._, tom. xvi. p. 50.]
[Footnote 20: See _Winter Birds of Florida_, p. 206, Table F.]
[Footnote 21: See Table I, p. 211, of Allen"s _Winter Birds of Florida_.]
[Footnote 22: _Proc. Zool. Soc._, 1864, p. 64.]
[Footnote 23: J.A. Allen, on Geographical Variation among North American Mammals, _Bull. U.S. Geol. and Geog. Survey_, vol. ii. p. 314 (1876).]
[Footnote 24: _Proc. Zool. Soc. Lond._, 1864, p. 700, and 1868, p. 28.]