Darwinism (1889) Part 3

[Footnote 15: The Kestrel, which usually feeds on mice, birds, and frogs, sometimes stays its hunger with earthworms, as do some of the American buzzards. The Honey-buzzard sometimes eats not only earthworms and slugs, but even corn; and the Buteo borealis of North America, whose usual food is small mammals and birds, sometimes eats crayfish.]

CHAPTER III

THE VARIABILITY OF SPECIES IN A STATE OF NATURE

Importance of variability--Popular ideas regarding it--Variability of the lower animals--The variability of insects--Variation among lizards--Variation among birds--Diagrams of bird-variation--Number of varying individuals--Variation in the mammalia--Variation in internal organs--Variations in the skull--Variations in the habits of Animals--The Variability of plants--Species which vary little--Concluding remarks.

The foundation of the Darwinian theory is the variability of species, and it is quite useless to attempt even to understand that theory, much less to appreciate the completeness of the proof of it, unless we first obtain a clear conception of the nature and extent of this variability.

The most frequent and the most misleading of the objections to the efficacy of natural selection arise from ignorance of this subject, an ignorance shared by many naturalists, for it is only since Mr. Darwin has taught us their importance that varieties have been systematically collected and recorded; and even now very few collectors or students bestow upon them the attention they deserve. By the older naturalists, indeed, varieties--especially if numerous, small, and of frequent occurrence--were looked upon as an unmitigated nuisance, because they rendered it almost impossible to give precise definitions of species, then considered the chief end of systematic natural history. Hence it was the custom to describe what was supposed to be the "typical form" of species, and most collectors were satisfied if they possessed this typical form in their cabinets. Now, however, a collection is valued in proportion as it contains ill.u.s.trative specimens of all the varieties that occur in each species, and in some cases these have been carefully described, so that we possess a considerable ma.s.s of information on the subject. Utilising this information we will now endeavour to give some idea of the nature and extent of variation in the species of animals and plants.

It is very commonly objected that the widespread and constant variability which is admitted to be a characteristic of domesticated animals and cultivated plants is largely due to the unnatural conditions of their existence, and that we have no proof of any corresponding amount of variation occurring in a state of nature. Wild animals and plants, it is said, are usually stable, and when variations occur these are alleged to be small in amount and to affect superficial characters only; or if larger and more important, to occur so rarely as not to afford any aid in the supposed formation of new species.

This objection, as will be shown, is utterly unfounded; but as it is one which goes to the very root of the problem, it is necessary to enter at some length into the various proofs of variation in a state of nature.

This is the more necessary because the materials collected by Mr. Darwin bearing on this question have never been published, and comparatively few of them have been cited in _The Origin of Species_; while a considerable body of facts has been made known since the publication of the last edition of that work.

_Variability of the Lower Animals_.

Among the lowest and most ancient marine organisms are the Foraminifera, little ma.s.ses of living jelly, apparently structureless, but which secrete beautiful sh.e.l.ly coverings, often perfectly symmetrical, as varied in form as those of the mollusca and far more complicated. These have been studied with great care by many eminent naturalists, and the late Dr. W.B. Carpenter in his great work--the _Introduction to the Study of the Foraminifera_--thus refers to their variability: "There is not a single species of plant or animal of which the range of variation has been studied by the collocation and comparison of so large a number of specimens as have pa.s.sed under the review of Messrs. Williamson, Parker, Rupert Jones, and myself in our studies of the types of this group;" and he states as the result of this extensive comparison of specimens: "The range of variation is so great among the Foraminifera as to include not merely those differential characters which have been usually accounted _specific_, but also those upon which the greater part of the _genera_, of this group have been founded, and even in some instances those of its _orders_."[16]

Coming now to a higher group--the Sea-Anemones--Mr. P.H. Gosse and other writers on these creatures often refer to variations in size, in the thickness and length of the tentacles, the form of the disc and of the mouth, and the character of surface of the column, while the colour varies enormously in a great number of the species. Similar variations occur in all the various groups of marine invertebrata, and in the great sub-kingdom of the mollusca they are especially numerous. Thus, Dr. S.P.

Woodward states that many present a most perplexing amount of variation, resulting (as he supposes) from supply of food, variety of depth and of saltness of the water; but we know that many variations are quite independent of such causes, and we will now consider a few cases among the land-mollusca in which they have been more carefully studied.

In the small forest region of Oahu, one of the Sandwich Islands, there have been found about 175 species of land-sh.e.l.ls represented by 700 or 800 varieties; and we are told by the Rev. J.T. Gulick, who studied them carefully, that "we frequently find a genus represented in several successive valleys by allied species, sometimes feeding on the same, sometimes on different plants. In every such case the valleys that are nearest to each other furnish the most nearly allied forms; _and a full set of the varieties of each species presents a minute gradation of forms between the more divergent types found in the more widely separated localities_."

In most land-sh.e.l.ls there is a considerable amount of variation in colour, markings, size, form, and texture or striation of the surface, even in specimens collected in the same locality. Thus, a French author has enumerated no less than 198 varieties of the common wood-snail (Helix nemoralis), while of the equally common garden-snail (Helix hortensis) ninety varieties have been described. Fresh-water sh.e.l.ls are also subject to great variation, so that there is much uncertainty as to the number of species; and variations are especially frequent in the Planorbidae, which exhibit many eccentric deviations from the usual form of the species--deviations which must often affect the form of the living animal. In Mr. Ingersoll's Report on the Recent Mollusca of Colorado many of these extraordinary variations are referred to, and it is stated that a sh.e.l.l (Helisonia trivolvis) abundant in some small ponds and lakes, had scarcely two specimens alike, and many of them closely resembled other and altogether distinct species.[17]

_The Variability of Insects_.

Among Insects there is a large amount of variation, though very few entomologists devote themselves to its investigation. Our first examples will be taken from the late Mr. T. Vernon Wollaston's book, _On the Variation of Species_, and they must be considered as indications of very widespread though little noticed phenomena. He speaks of the curious little carabideous beetles of the genus Notiophilus as being "extremely unstable both in their sculpture and hue;" of the common Calathus mollis as having "the hind wings at one time ample, at another rudimentary, and at a third nearly obsolete;" and of the same irregularity as to the wings being characteristic of many Orthoptera and of the h.o.m.opterous Fulgoridae. Mr. Westwood in his _Modern Cla.s.sification of Insects_ states that "the species of Gerris, Hydrometra, and Velia are mostly found perfectly apterous, though occasionally with full-sized wings."

It is, however, among the Lepidoptera (b.u.t.terflies and moths) that the most numerous cases of variation have been observed, and every good collection of these insects affords striking examples. I will first adduce the testimony of Mr. Bates, who speaks of the b.u.t.terflies of the Amazon valley exhibiting innumerable local varieties or races, while some species showed great individual variability. Of the beautiful Mechanitis Polymnia he says, that at Ega on the Upper Amazons, "it varies not only in general colour and pattern, but also very considerably in the shape of the wings, especially in the male s.e.x."

Again, at St. Paulo, Ithomia Orolina exhibits four distinct varieties, all occurring together, and these differ not only in colour but in form, one variety being described as having the fore wings much elongated in the male, while another is much larger and has "the hind wings in the male different in shape." Of Heliconius Numata Mr. Bates says: "This species is so variable that it is difficult to find two examples exactly alike," while "it varies in structure as well as in colours. The wings are sometimes broader, sometimes narrower; and their edges are simple in some examples and festooned in others." Of another species of the same genus, H. melpomene, ten distinct varieties are described all more or less connected by intermediate forms, and four of these varieties were obtained at one locality, Serpa on the north bank of the Amazon.

Ceratina Ninonia is another of these very unstable species exhibiting many local varieties which are, however, incomplete and connected by intermediate forms; while the several species of the genus Lycorea all vary to such an extent as almost to link them together, so that Mr.

Bates thinks they might all fairly be considered as varieties of one species only.

Turning to the Eastern Hemisphere we have in Papilio Severus a species which exhibits a large amount of simple variation, in the presence or absence of a pale patch on the upper wings, in the brown submarginal marks on the lower wings, in the form and extent of the yellow band, and in the size of the specimens. The most extreme forms, as well as the intermediate ones, are often found in one locality and in company with each other. A small b.u.t.terfly (Terias hecabe) ranges over the whole of the Indian and Malayan regions to Australia, and everywhere exhibits great variations, many of which have been described as distinct species; but a gentleman in Australia bred two of these distinct forms (T. hecabe and T. Aesiope), with several intermediates, from one batch of caterpillars found feeding together on the same plant.[18] It is therefore very probable that a considerable number of supposed distinct species are only individual varieties.

Cases of variation similar to those now adduced among b.u.t.terflies might be increased indefinitely, but it is as well to note that such important characters as the neuration of the wings, on which generic and family distinctions are often established, are also subject to variation. The Rev. R.P. Murray, in 1872, laid before the Entomological Society examples of such variation in six species of b.u.t.terflies, and other cases have been since described. The larvae of b.u.t.terflies and moths are also very variable, and one observer recorded in the _Proceedings of the Entomological Society for_ 1870 no less than sixteen varieties of the caterpillar of the bedstraw hawk-moth (Deilephela galii).

_Variation among Lizards_.

Pa.s.sing on from the lower animals to the vertebrata, we find more abundant and more definite evidence as to the extent and amount of individual variation. I will first give a case among the Reptilia from some of Mr. Darwin's unpublished MSS., which have been kindly lent me by Mr. Francis Darwin.

"M. Milne Edwards (_Annales des Sci. Nat._, I ser., tom. xvi. p. 50) has given a curious table of measurements of fourteen specimens of Lacerta muralis; and, taking the length of the head as a standard, he finds the neck, trunk, tail, front and hind legs, colour, and femoral pores, all varying wonderfully; and so it is more or less with other species. So apparently trifling a character as the scales on the head affording almost the only constant characters."

[Ill.u.s.tration: FIG. 1.--Variations of Lacerta muralis.]

[Ill.u.s.tration: FIG. 2.--Variation of Lizards.]

As the table of measurements above referred to would give no clear conception of the nature and amount of the variation without a laborious study and comparison of the figures, I have endeavoured to find a method of presenting the facts to the eye, so that they may be easily grasped and appreciated. In the diagram opposite, the comparative variations of the different organs of this species are given by means of variously bent lines. The head is represented by a straight line because it presented (apparently) no variation. The body is next given, the specimens being arranged in the order of their size from No. 1, the smallest, to No. 14, the largest, the actual lengths being laid down from a base line at a suitable distance below, in this case two inches below the centre, the mean length of the body of the fourteen specimens being two inches. The respective lengths of the neck, legs, and toe of each specimen are then laid down in the same manner at convenient distances apart for comparison; and we see that their variations bear no definite relation to those of the body, and not much to those of each other. With the exception of No. 5, in which all the parts agree in being large, there is a marked independence of each part, shown by the lines often curving in opposite directions; which proves that in those specimens one part is large while the other is small. The actual amount of the variation is very great, ranging from one-sixth of the mean length in the neck to considerably more than a fourth in the hind leg, and this among only fourteen examples which happen to be in a particular museum.

To prove that this is not an isolated case, Professor Milne Edwards also gives a table showing the amount of variation in the museum specimens of six common species of lizards, also taking the head as the standard, so that the comparative variation of each part to the head is given. In the accompanying diagram (Fig. 2) the variations are exhibited by means of lines of varying length. It will be understood that, however much the specimens varied in _size_, if they had kept the same _proportions_, the variation line would have been in every case reduced to a point, as in the neck of L. velox which exhibits no variation. The different proportions of the variation lines for each species may show a distinct mode of variation, or may be merely due to the small and differing number of specimens; for it is certain that whatever amount of variation occurs among a few specimens will be greatly increased when a much larger number of specimens are examined. That the amount of variation is large, may be seen by comparing it with the actual length of the head (given below the diagram) which was used as a standard in determining the variation, but which itself seems not to have varied.[19]

_Variation among Birds_.

Coming now to the cla.s.s of Birds, we find much more copious evidence of variation. This is due partly to the fact that Ornithology has perhaps a larger body of devotees than any other branch of natural history (except entomology); to the moderate size of the majority of birds; and to the circ.u.mstance that the form and dimensions of the wings, tail, beak, and feet offer the best generic and specific characters and can all be easily measured and compared. The most systematic observations on the individual variation of birds have been made by Mr. J.A. Allen, in his remarkable memoir: "On the Mammals and Winter Birds of East Florida, with an examination of certain a.s.sumed specific characters in Birds, and a sketch of the Bird Faunae of Eastern North America," published in the _Bulletin of the Museum of Comparative Zoology_ at Harvard College, Cambridge, Ma.s.sachusetts, in 1871. In this work exact measurements are given of all the chief external parts of a large number of species of common American birds, from twenty to sixty or more specimens of each species being measured, so that we are able to determine with some precision the nature and extent of the variation that usually occurs.

Mr. Allen says: "The facts of the case show that a variation of from 15 to 20 per cent in general size, and an equal degree of variation in the relative size of different parts, may be ordinarily expected among specimens of the same species and s.e.x, taken at the same locality, while in some cases the variation is even greater than this." He then goes on to show that each part varies to a considerable extent independently of the other parts; so that when the size varies, the proportions of all the parts vary, often to a much greater amount. The wing and tail, for example, besides varying in length, vary in the proportionate length of each feather, and this causes their outline to vary considerably in shape. The bill also varies in length, width, depth, and curvature. The tarsus varies in length, as does each toe separately and independently; and all this not to a minute degree requiring very careful measurement to detect it at all, but to an amount easily seen without any measurement, as it averages one-sixth of the whole length and often reaches one-fourth. In twelve species of common perching birds the wing varied (in from twenty-five to thirty specimens) from 14 to 21 per cent of the mean length, and the tail from 13.8 to 23.4 per cent. The variation of the form of the wing can be very easily tested by noting which feather is longest, which next in length, and so on, the respective feathers being indicated by the numbers 1, 2, 3, etc., commencing with the outer one. As an example of the irregular variation constantly met with, the following occurred among twenty-five specimens of Dendroeca coronata. Numbers bracketed imply that the corresponding feathers were of equal length.[20]

RELATIVE LENGTHS OF PRIMARY WING FEATHERS OF DENDROECA CORONATA.

---------+-----------+----------+-----------+----------+---------- Longest. | Second in | Third in | Fourth in | Fifth in | Sixth in | Length. | Length. | Length. | Length. | Length.

---------+-----------+----------+-----------+----------+---------- 2 | 3 | 1 | 4 | 5 | 6 3 | 2 | 4 | 1 | 5 | 6 | / 2 | | | | 3 | { | 1 | 5 | 6 | 7 | 4 | | | | 2 | | | | | } | 4 | 1 | 5 | 6 | 7 3 / | | | | | 2 | | | | | 1 | | | | | | } | 5 | 6 | 7 | 8 | 9 3 | | | | | | 4 / | | | | | ---------+-----------+----------+-----------+----------+----------

Here we have five very distinct proportionate lengths of the wing feathers, any one of which is often thought sufficient to characterise a distinct species of bird; and though this is rather an extreme case, Mr.

Allen a.s.sures us that "the comparison, extended in the table to only a few species, has been carried to scores of others with similar results."

Along with this variation in size and proportions there occurs a large amount of variation in colour and markings. "The difference in intensity of colour between the extremes of a series of fifty or one hundred specimens of any species, collected at a single locality, and nearly at the same season of the year, is often as great as occurs between truly distinct species." But there is also a great amount of individual variability in the markings of the same species. Birds having the plumage varied with streaks and spots differ exceedingly in different individuals of the same species in respect to the size, shape, and number of these marks, and in the general aspect of the plumage resulting from such variations. "In the common song sparrow (Melospiza melodia), the fox-coloured sparrow (Pa.s.serella iliaca), the swamp sparrow (Melospiza pal.u.s.tris), the black and white creeper (Mniotilta varia), the water-wagtail (Seiurus novaeboracencis), in t.u.r.dus fuscescens and its allies, the difference in the size of the streaks is often very considerable. In the song sparrow they vary to such an extent that in some cases they are reduced to narrow lines; in others 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."

Mr. Allen then goes on to particularise several species in which such variations occur, giving cases in which two specimens taken at the same place on the same day exhibited the two extremes of coloration. Another set of variations is thus described: "The white markings so common on the wings and tails of birds, as the bars formed by the white tips of the greater wing-coverts, the white patch occasionally present at the base of the primary quills, or the white band crossing them, and the white patch near the end of the outer tail-feathers are also extremely liable to variation in respect to their extent and the number of 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 s.e.x, 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_, ill.u.s.trating variations of size and proportion, more clearly before the reader, I have prepared a series of diagrams ill.u.s.trating the more important facts and their bearings on the Darwinian theory.

The first of these is intended, mainly, to show the actual amount of the 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.

[Ill.u.s.tration: 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 a.s.sume 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.

[Ill.u.s.tration: FIG. 4.--Dolichonyx oryzivorus. 20 Males.]

[Ill.u.s.tration: 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 pa.s.sing 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.