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

have been exported from the former country in a single year, their value

being £67,000. In both countries, sheep-runs have been greatly

deteriorated in value by the abundance of rabbits, which destroy the

herbage; and in some cases they have had to be abandoned altogether.]

usually two young produced, but it may be that in most cases only one of

these comes to maturity.]
[Footnote 12: _Origin of Species_, p. 59. Professor A. Newton, however,

informs me that these species do not interfere with one another in the

way here stated.]
[Footnote 13: Winwood Reade's _Martyrdom of Man,_ p. 520.]
[Footnote 14: _Nineteenth Century,_ February 1888, pp. 162, 163.]
[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

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 illustrative 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 mass 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.

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 masses of living jelly, apparently structureless, but which

secrete beautiful shelly 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 passed 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.

have been found about 175 species of land-shells 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_."

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 shells 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 shell (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_.

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 Homopterous Fulgoridae. Mr. Westwood in his _Modern

Classification of Insects_ states that "the species of Gerris,

Hydrometra, and Velia are mostly found perfectly apterous, though

occasionally with full-sized wings."

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 butterflies 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 sex."

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 butterfly (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.

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 butterflies, and other

cases have been since described. The larvae of butterflies 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_.
Passing 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."
[Illustration: FIG. 1.--Variations of Lacerta muralis.]
[Illustration: 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 class 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

circumstance 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 assumed 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, Massachusetts, 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 sex, 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 assures 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 (Passerella iliaca), the swamp

sparrow (Melospiza palustris), the black and white creeper (Mniotilta

varia), the water-wagtail (Seiurus novaeboracencis), in Turdus

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."

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