inherited from a remote period, since that period when the species first
branched off from their common progenitor, and subsequently have not varied
or come to differ in any degree, or only in a slight degree, it is not
probable that they should vary at the present day. On the other hand, the
points in which species differ from other species of the same genus, are
called specific characters; and as these specific characters have varied
and come to differ within the period of the branching off of the species
from a common progenitor, it is probable that they should still often be in
some degree variable,--at least more variable than those parts of the
organisation which have for a very long period remained constant.
In connexion with the present subject, I will make only two other remarks.
I think it will be admitted, without my entering on details, that secondary
sexual characters are very variable; I think it also will be admitted that
species of the same group differ from each other more widely in their
secondary sexual characters, than in other parts of their organisation;
compare, for instance, the amount of difference between the males of
gallinaceous birds, in which secondary sexual characters are strongly
displayed, with the amount of difference between their females; and the
truth of this proposition will be granted. The cause of the original
variability of secondary sexual characters is not manifest; but we can see
why these characters should not have been rendered as constant and uniform
as other parts of the organisation; for secondary sexual characters have
been accumulated by sexual selection, which is less rigid in its action
than ordinary selection, as it does not entail death, but only gives fewer
offspring to the less favoured males. Whatever the cause may be of the
variability of secondary sexual characters, as they are highly variable,
sexual selection will have had a wide scope for action, and may thus
readily have succeeded in giving to the species of the same group a greater
amount of difference in their sexual characters, than in other parts of
their structure.
It is a remarkable fact, that the secondary sexual differences between the
two sexes of the same species are generally displayed in the very same
parts of the organisation in which the different species of the same genus
differ from each other. Of this fact I will give in illustration two
instances, the first which happen to stand on my list; and as the
differences in these cases are of a very unusual nature, the relation can
hardly be accidental. The same number of joints in the tarsi is a
character generally common to very large groups of beetles, but in the
Engidae, as Westwood has remarked, the number varies greatly; and the
number likewise differs in the two sexes of the same species: again in
fossorial hymenoptera, the manner of neuration of the wings is a character
of the highest importance, because common to large groups; but in certain
genera the neuration differs in the different species, and likewise in the
two sexes of the same species. This relation has a clear meaning on my
view of the subject: I look at all the species of the same genus as having
as certainly descended from the same progenitor, as have the two sexes of
any one of the species. Consequently, whatever part of the structure of
the common progenitor, or of its early descendants, became variable;
variations of this part would it is highly probable, be taken advantage of
by natural and sexual selection, in order to fit the several species to
their several places in the economy of nature, and likewise to fit the two
sexes of the same species to each other, or to fit the males and females to
different habits of life, or the males to struggle with other males for the
possession of the females.
Finally, then, I conclude that the greater variability of specific
characters, or those which distinguish species from species, than of
generic characters, or those which the species possess in common;--that the
frequent extreme variability of any part which is developed in a species in
an extraordinary manner in comparison with the same part in its congeners;
and the not great degree of variability in a part, however extraordinarily
it may be developed, if it be common to a whole group of species;--that the
great variability of secondary sexual characters, and the great amount of
difference in these same characters between closely allied species;--that
secondary sexual and ordinary specific differences are generally displayed
in the same parts of the organisation,--are all principles closely
connected together. All being mainly due to the species of the same group
having descended from a common progenitor, from whom they have inherited
much in common,--to parts which have recently and largely varied being more
likely still to go on varying than parts which have long been inherited and
have not varied,--to natural selection having more or less completely,
according to the lapse of time, overmastered the tendency to reversion and
to further variability,--to sexual selection being less rigid than ordinary
selection,--and to variations in the same parts having been accumulated by
natural and sexual selection, and thus adapted for secondary sexual, and
for ordinary specific purposes.
Distinct species present analogous variations; and a variety of one species
often assumes some of the characters of an allied species, or reverts to
some of the characters of an early progenitor. -- These propositions will
be most readily understood by looking to our domestic races. The most
distinct breeds of pigeons, in countries most widely apart, present
sub-varieties with reversed feathers on the head and feathers on the
feet,--characters not possessed by the aboriginal rock-pigeon; these then
are analogous variations in two or more distinct races. The frequent
presence of fourteen or even sixteen tail-feathers in the pouter, may be
considered as a variation representing the normal structure of another
race, the fantail. I presume that no one will doubt that all such
analogous variations are due to the several races of the pigeon having
inherited from a common parent the same constitution and tendency to
variation, when acted on by similar unknown influences. In the vegetable
kingdom we have a case of analogous variation, in the enlarged stems, or
roots as commonly called, of the Swedish turnip and Ruta baga, plants which
several botanists rank as varieties produced by cultivation from a common
parent: if this be not so, the case will then be one of analogous
variation in two so-called distinct species; and to these a third may be
added, namely, the common turnip. According to the ordinary view of each
species having been independently created, we should have to attribute this
similarity in the enlarged stems of these three plants, not to the vera
causa of community of descent, and a consequent tendency to vary in a like
manner, but to three separate yet closely related acts of creation.
With pigeons, however, we have another case, namely, the occ
asional
appearance in all the breeds, of slaty-blue birds with two black bars on
the wings, a white rump, a bar at the end of the tail, with the outer
feathers externally edged near their bases with white. As all these marks
are characteristic of the parent rock-pigeon, I presume that no one will
doubt that this is a case of reversion, and not of a new yet analogous
variation appearing in the several breeds. We may I think confidently come
to this conclusion, because, as we have seen, these coloured marks are
eminently liable to appear in the crossed offspring of two distinct and
differently coloured breeds; and in this case there is nothing in the
external conditions of life to cause the reappearance of the slaty-blue,
with the several marks, beyond the influence of the mere act of crossing on
the laws of inheritance.
No doubt it is a very surprising fact that characters should reappear after
having been lost for many, perhaps for hundreds of generations. But when a
breed has been crossed only once by some other breed, the offspring
occasionally show a tendency to revert in character to the foreign breed
for many generations--some say, for a dozen or even a score of generations.
After twelve generations, the proportion of blood, to use a common
expression, of any one ancestor, is only 1 in 2048; and yet, as we see, it
is generally believed that a tendency to reversion is retained by this very
small proportion of foreign blood. In a breed which has not been crossed,
but in which both parents have lost some character which their progenitor
possessed, the tendency, whether strong or weak, to reproduce the lost
character might be, as was formerly remarked, for all that we can see to
the contrary, transmitted for almost any number of generations. When a
character which has been lost in a breed, reappears after a great number of
generations, the most probable hypothesis is, not that the offspring
suddenly takes after an ancestor some hundred generations distant, but that
in each successive generation there has been a tendency to reproduce the
character in question, which at last, under unknown favourable conditions,
gains an ascendancy. For instance, it is probable that in each generation
of the barb-pigeon, which produces most rarely a blue and black-barred
bird, there has been a tendency in each generation in the plumage to assume
this colour. This view is hypothetical, but could be supported by some
facts; and I can see no more abstract improbability in a tendency to
produce any character being inherited for an endless number of generations,
than in quite useless or rudimentary organs being, as we all know them to
be, thus inherited. Indeed, we may sometimes observe a mere tendency to
produce a rudiment inherited: for instance, in the common snapdragon
(Antirrhinum) a rudiment of a fifth stamen so often appears, that this
plant must have an inherited tendency to produce it.
As all the species of the same genus are supposed, on my theory, to have
descended from a common parent, it might be expected that they would
occasionally vary in an analogous manner; so that a variety of one species
would resemble in some of its characters another species; this other
species being on my view only a well-marked and permanent variety. But
characters thus gained would probably be of an unimportant nature, for the
presence of all important characters will be governed by natural selection,
in accordance with the diverse habits of the species, and will not be left
to the mutual action of the conditions of life and of a similar inherited
constitution. It might further be expected that the species of the same
genus would occasionally exhibit reversions to lost ancestral characters.
As, however, we never know the exact character of the common ancestor of a
group, we could not distinguish these two cases: if, for instance, we did
not know that the rock-pigeon was not feather-footed or turn-crowned, we
could not have told, whether these characters in our domestic breeds were
reversions or only analogous variations; but we might have inferred that
the blueness was a case of reversion, from the number of the markings,
which are correlated with the blue tint, and which it does not appear
probable would all appear together from simple variation. More especially
we might have inferred this, from the blue colour and marks so often
appearing when distinct breeds of diverse colours are crossed. Hence,
though under nature it must generally be left doubtful, what cases are
reversions to an anciently existing character, and what are new but
analogous variations, yet we ought, on my theory, sometimes to find the
varying offspring of a species assuming characters (either from reversion
or from analogous variation) which already occur in some other members of
the same group. And this undoubtedly is the case in nature.
A considerable part of the difficulty in recognising a variable species in
our systematic works, is due to its varieties mocking, as it were, some of
the other species of the same genus. A considerable catalogue, also, could
be given of forms intermediate between two other forms, which themselves
must be doubtfully ranked as either varieties or species; and this shows,
unless all these forms be considered as independently created species, that
the one in varying has assumed some of the characters of the other, so as
to produce the intermediate form. But the best evidence is afforded by
parts or organs of an important and uniform nature occasionally varying so
as to acquire, in some degree, the character of the same part or organ in
an allied species. I have collected a long list of such cases; but here,
as before, I lie under a great disadvantage in not being able to give them.
I can only repeat that such cases certainly do occur, and seem to me very
remarkable.
I will, however, give one curious and complex case, not indeed as affecting
any important character, but from occurring in several species of the same
genus, partly under domestication and partly under nature. It is a case
apparently of reversion. The ass not rarely has very distinct transverse
bars on its legs, like those on the legs of a zebra: it has been asserted
that these are plainest in the foal, and from inquiries which I have made,
I believe this to be true. It has also been asserted that the stripe on
each shoulder is sometimes double. The shoulder stripe is certainly very
variable in length and outline. A white ass, but not an albino, has been
described without either spinal or shoulder-stripe; and these stripes are
sometimes very obscure, or actually quite lost, in dark-coloured asses.
The koulan of Pallas is said to have been seen with a double
shoulder-stripe. The hemionus has no shoulder-stripe; but traces of it, as
stated by Mr. Blyth and others, occasionally appear: and I have been
informed by Colonel Poole that foals of this species are generally striped
on the legs, and faintly on the shoulder. The quagga, though so plainly
barred like a zebra over the body, is without bars on the legs; but Dr.
hocks.
With respect to the horse, I have collected cases in England of the spinal
stripe in horses of the most distinct breeds, and of all colours;
transverse bars on the legs are not rare in duns, mouse-duns, and in one
instance in a chestnut: a faint shoulder-stripe may sometimes be seen in
duns, and I have seen a trace in a bay horse. My son made a careful
examination and sketch for me of a dun Belgian cart-horse with a double
stripe on each shoulder and with leg-stripes; and a man, whom I can
implicitly trust, has examined for me a small dun Welch pony with three
short parallel stripes on each shoulder.
In the north-west part of India the Kattywar breed of horses is so
generally striped, that, as I hear from Colonel Poole, who examined the
breed for the Indian Government, a horse without stripes is not considered
as purely-bred. The spine is always striped; the legs are generally
barred; and the shoulder-stripe, which is sometimes double and sometimes
treble, is common; the side of the face, moreover, is sometimes striped.
The stripes are plainest in the foal; and sometimes quite disappear in old
horses. Colonel Poole has seen both gray and bay Kattywar horses striped
when first foaled. I have, also, reason to suspect, from information given
me by Mr. W. W. Edwards, that with the English race-horse the spinal stripe
is much commoner in the foal than in the full-grown animal. Without here
entering on further details, I may state that I have collected cases of leg
and shoulder stripes in horses of very different breeds, in various
countries from Britain to Eastern China; and from Norway in the north to
the Malay Archipelago in the south. In all parts of the world these
stripes occur far oftenest in duns and mouse-duns; by the term dun a large
range of colour is included, from one between brown and black to a close
approach to cream-colour.
I am aware that Colonel Hamilton Smith, who has written on this subject,
believes that the several breeds of the horse have descended from several
aboriginal species--one of which, the dun, was striped; and that the
above-described appearances are all due to ancient crosses with the dun
stock. But I am not at all satisfied with this theory, and should be loth
to apply it to breeds so distinct as the heavy Belgian cart-horse, Welch
ponies, cobs, the lanky Kattywar race, &c., inhabiting the most distant
parts of the world.
Now let us turn to the effects of crossing the several species of the
horse-genus. Rollin asserts, that the common mule from the ass and horse
is particularly apt to have bars on its legs. I once saw a mule with its
legs so much striped that any one at first would have thought that it must
have been the product of a zebra; and Mr. W. C. Martin, in his excellent
treatise on the horse, has given a figure of a similar mule. In four
coloured drawings, which I have seen, of hybrids between the ass and zebra,
the legs were much more plainly barred than the rest of the body; and in
one of them there was a double shoulder-stripe. In Lord Moreton's famous
hybrid from a chestnut mare and male quagga, the hybrid, and even the pure
offspring subsequently produced from the mare by a black Arabian sire, were
much more plainly barred across the legs than is even the pure quagga.
Lastly, and this is another most remarkable case, a hybrid has been figured
by Dr. Gray (and he informs me that he knows of a second case) from the ass
and the hemionus; and this hybrid, though the ass seldom has stripes on its
legs and the hemionus has none and has not even a shoulder-stripe,
nevertheless had all four legs barred, and had three short
shoulder-stripes, like those on the dun Welch pony, and even had some
zebra-like stripes on the sides of its face. With respect to this last
The Origin of Species Page 18
