Darwinism (1889) Part 4

[Ill.u.s.tration: FIG. 6.--Cardinalis virginia.n.u.s. 31 Males.]

In the next diagram (Fig. 5) we have the variations in forty males of the Red-winged Blackbird (Agelaeus phoeniceus), and here we see the same general features. One-fifth of the whole number of specimens offer a large amount of variation either below or above the mean; while the wings, tail, and head vary quite independently of the body. The wing and tail too, though showing some amount of correlated variation, yet in no less than nine cases vary in opposite directions as compared with the preceding species.

The next diagram (Fig. 6), showing the variations of thirty-one males of the Cardinal bird (Cardinalis virginia.n.u.s), exhibits these features much more strongly. The amount of variation in proportion to the size of the bird is very much greater; while the variations of the wing and tail not only have no correspondence with that of the body but very little with each other. In no less than twelve or thirteen instances they vary in opposite directions, while even where they correspond in direction the amount of the variation is often very disproportionate.

As the proportions of the tarsi and toes of birds have great influence on their mode of life and habits and are often used as specific or even generic characters, I have prepared a diagram (Fig. 7) to show the variation in these parts only, among twenty specimens of each of four species of birds, four or five of the most variable alone being given.

The extreme divergence of each of the lines in a vertical direction shows the actual amount of variation; and if we consider the small length of the toes of these small birds, averaging about three-quarters of an inch, we shall see that the variation is really very large; while the diverging curves and angles show that each part varies, to a great extent, independently. It is evident that if we compared some thousands of individuals instead of only twenty, we should have an amount of independent variation occurring each year which would enable almost any modification of these important organs to be rapidly effected.

[Ill.u.s.tration: FIG. 7.--Variation of Tarsus and Toes.]

[Ill.u.s.tration: FIG. 8.--Variation of Birds in Leyden Museum.]

In order to meet the objection that the large amount of variability here shown depends chiefly on the observations of one person and on the birds of a single country, I have examined Professor Schlegel's Catalogue of the Birds in the Leyden Museum, in which he usually gives the range of variation of the specimens in the museum (which are commonly less than a dozen and rarely over twenty) as regards some of their more important dimensions. These fully support the statement of Mr. Allen, since they show an equal amount of variability when the numbers compared are sufficient, which, however, is not often the case. The accompanying diagram exhibits the actual differences of size in five organs which occur in five species taken almost at random from this catalogue. Here, again, we perceive that the variation is decidedly large, even among a very small number of specimens; while the facts all show that there is no ground whatever for the common a.s.sumption that natural species consist of individuals which are nearly all alike, or that the variations which occur are "infinitesimal" or even "small."

_The proportionate Number of Individuals which present a considerable amount of Variation._

The notion that variation is a comparatively exceptional phenomenon, and that in any case considerable variations occur very rarely in proportion to the number of individuals which do not vary, is so deeply rooted that it is necessary to show by every possible method of ill.u.s.tration how completely opposed it is to the facts of nature. I have therefore prepared some diagrams in which each of the individual birds measured is represented by a spot, placed at a proportionate distance, right and left, from the median line accordingly as it varies in excess or defect of the mean length as regards the particular part compared. As the object in this set of diagrams is to show the number of individuals which vary considerably in proportion to those which vary little or not at all, the scale has been enlarged in order to allow room for placing the spots without overlapping each other.

In the diagram opposite twenty males of Icterus Baltimore are registered, so as to exhibit to the eye the proportionate number of specimens which vary, to a greater or less amount, in the length of the tail, wing, tarsus, middle toe, hind toe, and bill. It will be noticed that there is usually no very great acc.u.mulation of dots about the median line which shows the average dimensions, but that a considerable number are spread at varying distances on each side of it.

In the next diagram (Fig. 10), showing the variation among forty males of Agelaeeus phoeniceus, this approach to an equable spreading of the variations is still more apparent; while in Fig. 12, where fifty-eight specimens of Cardinalis virginia.n.u.s are registered, we see a remarkable spreading out of the spots, showing in some of the characters a tendency to segregation into two or more groups of individuals, each varying considerably from the mean.

[Ill.u.s.tration: FIG. 9]

[Ill.u.s.tration: FIG. 10.]

[Ill.u.s.tration: FIG. 11.]

In order fully to appreciate the teaching of these diagrams, we must remember, that, whatever kind and amount of variations are exhibited by the few specimens here compared, would be greatly extended and brought into symmetrical form if large numbers--thousands or millions--were subjected to the same process of measurement and registration. We know, from the general law which governs variations from a mean value, that with increasing numbers the range of variation of each part would increase also, at first rather rapidly and then more slowly; while gaps and irregularities would be gradually filled up, and at length the distribution of the dots would indicate a tolerably regular curve of double curvature like those shown in Fig. 11. The great divergence of the dots, when even a few specimens are compared, shows that the curve, with high numbers, would be a flat one like the lower curve in the ill.u.s.tration here given. This being the case it would follow that a very large proportion of the total number of individuals const.i.tuting a species would diverge considerably from its average condition as regards each part or organ; and as we know from the previous diagrams of variation (Figs. 1 to 7) that each part varies to a considerable extent, _independently_, the materials constantly ready for natural selection to act upon are abundant in quant.i.ty and very varied in kind. Almost any combination of variations of distinct parts will be available, where required; and this, as we shall see further on, obviates one of the most weighty objections which have been urged against the efficiency of natural selection in producing new species, genera, and higher groups.

[Ill.u.s.tration: FIG. 12.]

_Variation in the Mammalia._

Owing to the generally large size of this cla.s.s of animals, and the comparatively small number of naturalists who study them, large series of specimens are only occasionally examined and compared, and thus the materials for determining the question of their variability in a state of nature are comparatively scanty. The fact that our domestic animals belonging to this group, especially dogs, present extreme varieties not surpa.s.sed even by pigeons and poultry among birds, renders it almost certain that an equal amount of variability exists in the wild state; and this is confirmed by the example of a species of squirrel (Sciurus carolinensis), of which sixteen specimens, all males and all taken in Florida, were measured and tabulated by Mr. Allen. The diagram here given shows, that, both the general amount of the variation and the independent variability of the several members of the body, accord completely with the variations so common in the cla.s.s of birds; while their amount and their independence of each other are even greater than usual.

_Variation in the Internal Organs of Animals._

In case it should be objected that the cases of variation hitherto adduced are in the external parts only, and that there is no proof that the internal organs vary in the same manner, it will be advisable to show that such varieties also occur. It is, however, impossible to adduce the same amount of evidence in this cla.s.s of variation, because the great labour of dissecting large numbers of specimens of the same species is rarely undertaken, and we have to trust to the chance observations of anatomists recorded in their regular course of study.

It must, however, be noted that a very large proportion of the variations already recorded in the external parts of animals necessarily imply corresponding internal variations. When feet and legs vary in size, it is because the bones vary; when the head, body, limbs, and tail change their proportions, the bony skeleton must also change; and even when the wing or tail feathers of birds become longer or more numerous, there is sure to be a corresponding change in the bones which support and the muscles which move them. I will, however, give a few cases of variations which have been directly observed.

[Ill.u.s.tration: FIG. 13.--Sciurus carolinensis. 32 specimens. Florida.]

Mr. Frank E. Beddard has kindly communicated to me some remarkable variations he has observed in the internal organs of a species of earthworm (Perionyx excavatus). The normal characters of this species are--

Setae forming a complete row round each segment.

Two pairs of spermathecae--spherical pouches without diverticulae--in segments 8 and 9.

Two pairs of testes in segments 11 and 12.

Ovaries, a single pair in segment 13.

Oviducts open by a common pore in the middle of segment 14.

Vasa deferentia open separately in segment 18, each furnished at its termination with a large prostate gland.

Between two and three hundred specimens were examined, and among them thirteen specimens exhibited the following marked variations:--

(1) The number of the spermathecae varied from two to three or four pairs, their position also varying.

(2) There were occasionally two pairs of ovaries, each with its own oviduct; the external apertures of these varied in position, being upon segments 13 and 14, 14 and 15, or 15 and 16.

Occasionally when there was only the normal single oviduct pore present it varied in position, once occurring on the 10th, and once on the 11th segment.

(3) The male generative pores varied in position from segments 14 to 20. In one instance there were two pairs instead of the normal single pair, and in this case each of the four apertures had its own prostate gland.

Mr. Beddard remarks that all, or nearly all, the above variations are found _normally_ in other genera and species.

When we consider the enormous number of earthworms and the comparatively very small number of individuals examined, we may be sure, not only that such variations as these occur with considerable frequency, but also that still more extraordinary deviations from the normal structure may often exist.

The next example is taken from Mr. Darwin's unpublished MSS.

"In some species of Shrews (Sorex) and in some field-mice (Arvicola), the Rev. L. Jenyns (_Ann. Nat. Hist._, vol. vii. pp.

267, 272) found the proportional length of the intestinal ca.n.a.l to vary considerably. He found the same variability in the number of the caudal vertebrae. In three specimens of an Arvicola he found the gall-bladder having a very different degree of development, and there is reason to believe it is sometimes absent. Professor Owen has shown that this is the case with the gall-bladder of the giraffe."

Dr. Crisp (_Proc. Zool. Soc._, 1862, p. 137) found the gall-bladder present in some specimens of Cervus superciliaris while absent in others; and he found it to be absent in three giraffes which he dissected. A double gall-bladder was found in a sheep, and in a small mammal preserved in the Hunterian Museum there are three distinct gall-bladders.

The length of the alimentary ca.n.a.l varies greatly. In three adult giraffes described by Professor Owen it was from 124 to 136 feet long; one dissected in France had this ca.n.a.l 211 feet long; while Dr. Crisp measured one of the extraordinary length of 254 feet, and similar variations are recorded in other animals.[22]

The number of ribs varies in many animals. Mr. St. George Mivart says: "In the highest forms of the Primates, the number of true ribs is seven, but in Hylobates there are sometimes eight pairs. In Semnopithecus and Colobus there are generally seven, but sometimes eight pairs of true ribs. In the Cebidae there are generally seven or eight pairs, but in Ateles sometimes nine" (_Proc. Zool. Soc._, 1865, p. 568). In the same paper it is stated that the number of dorsal vertebrae in man is normally twelve, very rarely thirteen. In the Chimpanzee there are normally thirteen dorsal vertebrae, but occasionally there are fourteen or only twelve.

_Variations in the Skull._

[Ill.u.s.tration: FIG. 14.--Variation of Skull of Wolf. 10 specimens.]

Among the nine adult male Orang-utans, collected by myself in Borneo, the skulls differed remarkably in size and proportions. The orbits varied in width and height, the cranial ridge was either single or double, either much or little developed, and the zygomatic aperture varied considerably in size. I noted particularly that these variations bore no necessary relation to each other, so that a large temporal muscle and zygomatic aperture might exist either with a large or a small cranium; and thus was explained the curious difference between the single-crested and the double-crested skulls, which had been supposed to characterise distinct species. As an instance of the amount of variation in the skulls of fully adult male orangs, I found the width between the orbits externally to be only 4 inches in one specimen and fully 5 inches in another.