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 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.
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 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.
In the small forest region of Oahu, one of the Sandwich Islands, there
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_."
In most land-shells 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 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_.
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 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."
It is, however, among the Lepidoptera (butterflies 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 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.
Cases of variation similar to those now adduced among butterflies 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 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."
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 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.
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.
[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 next diagram (Fig. 6), showing the variations of thirty-one males of
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.
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.
[Illustration: FIG. 7.--Variation of Tarsus and Toes.]
[Illustration: 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 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._
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 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.
[Illustration: FIG. 12.]
_Variation in the Mammalia._
Owing to the generally large size of this class 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
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.
_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 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.
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.
[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--
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 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.
The length of the alimentary canal varies greatly. In three adult
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]
[Illustration: FIG. 15.--Variation of 8 skulls (Ursus labiatus).]
[Illustration: 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 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
help of an ordinary divided rule or a pair of compasses 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 illustrating 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
illustrating 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-grass (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 class 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 pubescent 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 Müller. 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 illustration 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. pyrenaicum (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 Genève_, 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 class to which this family belongs
the unilocular ovarium is free, but M. Dubury (_Mem. Soc. Phys. de
Genève_, 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 Gynobase, _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: 'Voilà
donc dans un même individu des loges et un style qui se rattachent
tantôt a un axe vertical, et tantôt a un gynobase; donc celui-ci n'est
qu'un axe veritable; mais cet axe est deprimé au lieu d'être 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
'Pleurorhizée' or 'Notor-hizée'; so again (p. 400) in Cochlearia
saxatilis M. Gay examined twenty-nine embryos, and of these sixteen were
vigorously 'pleurorhizées,' nine had characters intermediate between
pleuro-and notor-hizées, and four were pure notor-hizées."
"M. Raspail asserts (_Ann. des Scien. Nat._, ser. i. tom. v. p. 440)
that a grass (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 class, 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.]
[Footnote 25: See _Trans. Entomological Society of London_, 1887, p.
24.]
[Footnote 26: _Nature_, vol. xix. p. 554.]
[Footnote 27: _Nature_, vol. xvi. p. 163; and vol. xi. p. 227.]
[Footnote 28: _Ibid._, vol. xxxi. (1885), p. 533.]
[Footnote 29: _Nature_, vol. xxvi. p. 81.]
[Footnote 30: _Animals and Plants under Domestication_, vol. ii. p.
258.]