The Project Gutenberg ebook of Darwinism (1889), by Alfred Russel Wallace

feathers to which, in the same species, these markings extend." It is to

be especially noted that all these varieties are distinct from those

which depend on season, on age, or on sex, and that they are such as

have in many other species been considered to be of specific value.
These variations of colour could not be presented to the eye without a

series of carefully engraved plates, but in order to bring Mr. Allen's

_measurements_, illustrating variations of size and proportion, more

clearly before the reader, I have prepared a series of diagrams

illustrating the more important facts and their bearings on the

Darwinian theory.

variation, as it gives the true length of the wing and tail in the

extreme cases among thirty specimens of each of three species. The

shaded portion shows the minimum length, the unshaded portion the

additional length in the maximum. The point to be specially noted here

is, that in each of these common species there is about the same amount

of variation, and that it is so great as to be obvious at a glance.
[Illustration: FIG. 3.--Variation of Wings and Tail.]
There is here no question of "minute" or "infinitesimal" variation,

which many people suppose to be the only kind of variation that exists.

It cannot even be called small; yet from all the evidence we now possess

it seems to be the amount which characterises most of the common species

of birds.
It may be said, however, that these are the extreme variations, and only

occur in one or two individuals, while the great majority exhibit little

or no difference. Other diagrams will show that this is not the case;

but even if it were so, it would be no objection at all, because these

are the extremes among thirty specimens only. We may safely assume that

these thirty specimens, taken by chance, are not, in the case of all

these species, exceptional lots, and therefore we might expect at least

two similarly varying specimens in each additional thirty. But the

number of individuals, even in a very rare species, is probably thirty

thousand or more, and in a common species thirty, or even three hundred,

millions. Even one individual in each thirty, varying to the amount

shown in the diagram, would give at least a million in the total

population of any common bird, and among this million many would vary

much more than the extreme among thirty only. We should thus have a vast

body of individuals varying to a large extent in the length of the wings

and tail, and offering ample material for the modification of these

organs by natural selection. We will now proceed to show that other

parts of the body vary, simultaneously, but independently, to an equal

amount.
[Illustration: FIG. 4.--Dolichonyx oryzivorus. 20 Males.]
[Illustration: FIG. 5.--Agelaeus phoeniceus. 40 Males.]
The first bird taken is the common Bob-o-link or Rice-bird (Dolichonyx

oryzivorus), and the Diagram, Fig. 4, exhibits the variations of seven

important characters in twenty male adult specimens.[21] These

characters are--the lengths of the body, wing, tail, tarsus, middle toe,

outer toe, and hind toe, being as many as can be conveniently exhibited

in one diagram. The length of the body is not given by Mr. Allen, but as

it forms a convenient standard of comparison, it has been obtained by

deducting the length of the tail from the total length of the birds as

given by him. The diagram has been constructed as follows:--The twenty

specimens are first arranged in a series according to the body-lengths

(which may be considered to give the size of the bird), from the

shortest to the longest, and the same number of vertical lines are

drawn, numbered from one to twenty. In this case (and wherever

practicable) the body-length is measured from the lower line of the

diagram, so that the actual length of the bird is exhibited as well as

the actual variations of length. These can be well estimated by means of

the horizontal line drawn at the mean between the two extremes, and it

will be seen that one-fifth of the total number of specimens taken on

either side exhibits a very large amount of variation, which would of

course be very much greater if a hundred or more specimens were

compared. The lengths of the wing, tail, and other parts are then laid

down, and the diagram thus exhibits at a glance the comparative

variation of these parts in every specimen as well as the actual amount

of variation in the twenty specimens; and we are thus enabled to arrive

at some important conclusions.
We note, first, that the variations of none of the parts follow the

variations of the body, but are sometimes almost in an opposite

direction. Thus the longest wing corresponds to a rather small body, the

longest tail to a medium body, while the longest leg and toes belong to

only a moderately large body. Again, even related parts do not

constantly vary together but present many instances of independent

variation, as shown by the want of parallelism in their respective

variation-lines. In No. 5 (see Fig. 4) the wing is very long, the tail

moderately so; while in No. 6 the wing is much shorter while the tail is

considerably longer. The tarsus presents comparatively little variation;

and although the three toes may be said to vary in general together,

there are many divergencies; thus, in passing from No. 9 to No. 10, the

outer toe becomes longer, while the hind toe becomes considerably

shorter; while in Nos. 3 and 4 the middle toe varies in an opposite way

to the outer and the hind toes.
[Illustration: FIG. 6.--Cardinalis virginianus. 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 Cardinal bird (Cardinalis virginianus), 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.

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.

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 assumption 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._

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 illustration 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 accumulation 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 virginianus 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.
[Illustration: FIG. 9]
[Illustration: FIG. 10.]
[Illustration: 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

illustration here given. This being the case it would follow that a very

large proportion of the total number of individuals constituting 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 quantity 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.

_Variation in the Mammalia._

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

surpassed 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 class of birds; while

their amount and their independence of each other are even greater than

usual.

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 class 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.

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.
[Illustration: 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--

diverticulae--in segments 8 and 9.

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.

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.

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.

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 canal

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.

giraffes described by Professor Owen it was from 124 to 136 feet long;

one dissected in France had this canal 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._
[Illustration: 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.
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 labiatus), 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]

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 illustrate 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