Charles Darwin.

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for secondary sexual, and for ordinary purposes.


These propositions will be most readily understood by looking to our
domestic races. The most distinct breeds of the pigeon, in countries widely
apart, present sub-varieties with reversed feathers on the head, and with
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 as commonly called
roots, 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 in-
dependently 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 consequent tendency to vary in a like manner, but to three
separate yet closely related acts of creation. Many similar cases of analogous
variation have been observed by Naudin in the great gourd family, and by
various authors in our cereals. Similar cases occurring with insects under
natural conditions have lately been discussed with much ability by Mr.
Walsh, who has grouped them under his law of equal variability.

With pigeons, however, we have another case, namely, the occasional
appearance in all the breeds, of slaty-blue birds with two black bars on the
wings, white loins, a bar at the end of the tail, with the outer feathers
externally edged near their basis with white. As all these marks are charac-
teristic 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 appear-
ing in the several breeds. We may, I think, confidently come to this conclu-
sion, because, as we have seen, these colored marks are eminently liable to
appear in the crossed off"spring of two distinct and differently colored
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^ probably for hundreds of generations. But when


a breed has been crossed only once by some other breed, the offspring
occasionally show for many generations a tendency to revert in character to
the foreign breed — some say, for a dozen or even a score of generations.
After twelve generations, the proportion of blood, to use a common expres-
sion, from one ancestor, is only one in 2048; and yet, as we see, it is generally
believed that a tendency to reversion is retained by this remnant 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, as was
formerly remarked, for all that we can see to the contrary, be 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 f
probable hypothesis is, not that one individual suddenly takes after an
ancestor removed by some hundred generations, but that in each successive
generation the character in question has been lying latent, and at last,
under unknown favorable conditions, is developed. With the barb-pigeon,
for instance, which very rarely produces a blue bird, it is probable that
there is a latent tendency in each generation to produce blue plumage. The
abstract improbability of such a tendency being transmitted through a vast
number of generations, is not greater than that of quite useless or rudi-
mentary organs being similarly transmitted. A mere tendency to produce a
rudiment is indeed sometimes thus inherited.

As all the species of the same genus are supposed to be descended from a
common progenitor, it might be expected that they would occasionally vary
in an analogous manner; so that the varieties of two or more species would
resemble each other, or that a variety of one species would resemble in
certain characters another and distinct species, this other species being,
according to our view, only a well-marked and permanent variety. But
characters exclusively due to analogous variation would probably be on an
unimportant nature, for the preservation of all functionally important char-
acters will have been determined through natural selection, in accordance
with the different habits of the species. It might further be expected that
the species of the same genus would occasionally exhibit reversions to long-
lost characters. As, however, we do not know the common ancestor of any
natural group, we cannot distinguish between reversionary and analogous
characters. If, for instance, we did not know that the parent rock-pigeon
was not feather-footed or turn-crowned, we could not have told, whether
such characters in our domestic breeds were reversions or only analogous
variations; but we might have inferred that the blue color was a case of
reversion from the number of the markings, which are correlated with this
tint, and which would not probably have all appeared together from simple
variation. More especially we might have inferred this from the blue color
and the several marks so often appearing when differently colored breeds
are crossed. Hence, although under nature it must generally be left doubt-
ful, what cases are reversions to formerly existing characters, and what are
new but analogous variations, yet we ought, on our theory, sometimes to


find the varying oflfspring of a species assuming characters which are already
present in other members of the same group. And this undoubtedly is

the case.

The difficulty in distinguishing variable species is largely due to the
varieties mocking, as it were, other species of the same genus. A con-
siderable catalogue, also, could be given of forms intermediate between
two other forms, which themselves can only doubtfully be ranked as
species; and this shows, unless all these closely allied forms be considered
as independently created species, that they have in varying assumed some
of the characters of the others. But the best evidence of analogous variations
is afforded by parts or organs which are generally constant in character,
but which occasionally vary so as to resemble, in some degree, the same
part or organ in an allied species. I have collected a long Hst of such cases;
but here, as before, I lie under the great disadvantage of not being able to
give them. I can only repeat that such cases certainly occur, and seem to
me very remarkable.

I will, however, give one curious and complex case, not indeed as affect-
ing any important character, but from occurring in several species of the
same genus, partly under domestication and partly under nature. It is a
case almost certainly of reversion. The ass sometimes has very distinct trans-
verse bars on its legs, like those on the legs of the 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. The stripe on the shoulder is sometimes double,
and is very variable in length and outUne. A white ass, but not sm albino,
has been described without either spinal or shoulder-stripe; and these
stripes are sometimes very obscure, or actually quite lost, in dark-colored
asses. The koulan of Pallas is said to have been seen with a double shoulder-
stripe. Mr. Blyth has seen a specimen of the hemionus with a distinct
shoulder-stripe, though it properly has none; and I have been informed by
Colonel Poole that the 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. Gray has figured
one specimen with very distinct zebra-like bars on the 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 colors; 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 exaniination and
sketch for me of a dun Belgian cart-horse with a double stripe on each
shoulder and with leg-stripes. I have myself seen a dun Devonshire pony,
and a small dun Welsh pony has been carefully described to me, both with
three parallel stripes on each shoulder.

In the north-west part of India the Kattywar breed of horses is so gen-
erally striped, that, as I hear from Colonel Poole, who examined this 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 often 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. I have myself
recently bred a foal from a bay mare (offspring of a Turkoman horse and a
Flemish mare) by a bay English race-horse. This foal, when a week old,
was marked on its hinder quarters and on its forehead with numerous very
narrow, dark, zebra-like bars, and its legs were feebly striped. All the stripes
soon disappeared completely. 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 color is included, from one between
brown and black to a close approach to cream color.

I am aware that Colonel Hamilton Smith, who has written on this sub-
ject, believes that the several breeds of the horse are 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 this view may be safely rejected, for it is highly improbable that
the heavy Belgian cart-horse, Welsh ponies, Norwegian cobs, the lanky
Kattywar race, etc., inhabiting the most distant parts of the world, should
all have been crossed with one supposed aboriginal stock.

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 ; according to Mr. Gosse, in certain
parts of the United States, about nine out of ten mules have striped legs. I
once saw a mule with its legs so much striped that any one might have
thought that it was a hybrid zebra; and Mr. W. C. Martin, in his excellent
treatise on the horse, has given a figure of a similar mule. In four colored
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 Morton's famous
hybrid, from a chestnut mare and male quagga, the hybrid and even the
pure offspring subsequently produced from the same 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 only
occasionally has stripes on his 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 Devonshire and Welsh ponies,
and even had some zebra-like stripes on the sides of its face. With respect


to this last factj I was so convinced that not even a stripe of color appears
from what is commonly called chance, that I was led solely from the occur-
rence of the face-stripes on this hybrid from the ass and hemionus to ask
Colonel Poole whether such face-stripes ever occurred in the eminently
striped Kattywar breed of horses, and was, as we have seen, answered in
the affirmative.

What now are we to say to these several facts? We see several distinct
species of the horse genus becoming, by simple variation, striped on the legs
like a zebra, or striped on the shoulders like an ass. In the horse we see this
tendency strong whenever a dun tint appears — a tint which approaches to
that of the general coloring of the other species of the genus. The appear-
ance of the stripes is not accompanied by any change of form, or by any
other new character. We see this tendency to become striped most strongly
displayed in hybrids from between several of the most distinct species. Now
observe the case of the several breeds of pigeons: they are descended from
a pigeon (including two or three sub-species or geographical races) of a
bluish color, with certain bars and other marks; and when any breed as-
sumes by simple variation a bluish tint, these bars and other marks in-
variably reappear; but without any other change of form or character.
When the oldest and truest breeds of various colors are crossed, we see a
strong tendency for the blue tint and bars and marks to reappear in the
mongrels. I have stated that the most probable hypothesis to account for
the reappearance of very ancient characters, is — that there is a tendency
in the young of each successive generation to produce the long-lost charac-
ter, and that this tendency, from unknown causes, sometimes prevails. And
we have just seen that in several species of the horse genus the stripes are
either plainer or appear more commonly in the young than in the old. Call
the breeds of pigeons, some of which have bred true for centuries, species;
and how exactly parallel is the case with that of the species of the horse
genus! For myself, I venture confidently to look back thousands on thou-
sands of generations, and I see an animal striped like a zebra, but perhaps
otherwise very differently constructed, the common parent of our domestic
horse (whether or not it be descended from one or more wild stocks), of the
ass, the hemionus, quagga, and zebra.

He who believes that each equine species was independently created,
will, I presume, assert that each species has been created with a tendency
to vary, both under nature and under domestication, in this particular
manner, so as often to become striped like the other species of the genus;
and that each has been created with a strong tendency, when crossed with
species inhabiting distant quarters of the world, to produce hybrids resem-
bling in their stripes, not their own parents, but other species of the genus.
To admit this view, is, as it seems to me, to reject a real for an unreal, or
at least for an unknown cause. It makes the works of God a mere mockery
and deception; I would almost as soon believe, with the old and ignorant
cosmogonists, that fossil shells had never lived, but had been created in
stone so as to mock the shells living on the seashore.



Our ignorance of the laws of variation is profound. Not in one case out
of a hundred can we pretend to assign any reason why this or that part has
varied. But whenever we have the means of instituting a comparison, the
same laws appear to have acted in producing the lesser differences between
varieties of the same species, and the greater differences between species of
the same genus. Changed conditions generally induce mere fluctuating
variability, but sometimes they cause direct and definite effects; and these
may become strongly marked in the course of time, though we have not
sufficient evidence on this head. Habit in producing constitutional peculiari-
ties, and use in strengthening and disuse in weakening and diminishing
organs, appear in many cases to have been potent in their effects. Homologous
parts tend to vary in the same manner, and homologous parts tend to
cohere. Modifications in hard parts and in external parts sometimes affect
softer and internal parts. When one part is largely developed, perhaps it
tends to draw nourishment from the adjoining parts; and every part of the
structure which can be saved without detriment will be saved. Changes of
structure at an early age may affect parts subsequently developed; and
many cases of correlated variation, the nature of which we are unable to
understand, undoubtedly occur. Multiple parts are variable in number and
in structure, perhaps arising from such parts not having been closely spe-
cialized for any particular function, so that their modifications have not
been closely checked by natural selection. It follows probably from this same
cause, that organic beings low in the scale are more variable than those
standing higher in the scale, and which have their whole organization more
specialized. Rudimentary organs, from being useless, are not regulated by
natural selection, and hence are variable. Specific characters — that is, the
characters which have come to differ since the several species of the same
genus branched off from a common parent — are more variable than generic
characters, or those which have long been inherited, and have not differed
within this same period. In these remarks we have referred to special parts
or organs being still variable, because they have recently varied and thus
come to differ; but we have also seen in the second chapter that the same
principle applies to the whole individual; for in a district where many
species of a genus are found — that is, where there has been much former
variation and differentiation, or where the manufactory of new specific
forms has been actively at work — in that district and among these species,
we now find, on an average, most varieties. Secondary sexual characters
are highly variable, and such characters differ ranch in the species of the
same group. Variability in the same parts of the organization has generally
been taken advantage of in giving secondary sexual differences to the two
sexes of the same species, and specific differences to the several species of
the same genus. Any part or organ developed to an extraordinary size or in
an extraordinary manner, in comparison with the same part or organ in the


allied species, must have gone through an extraordinary amount of modifi-
cation since the genus arose; and thus we can understand why it should
often still be variable in a much higher degree than other parts; for varia-
tion is a long-continued and slow process, and natural selection will in
such cases not as yet have had time to overcome the tendency to further
variability and to reversion to a less modified state. But when a species with
an extraordinarily developed organ has become the parent of many modified
descendants — which in our view must be a very slow process, requiring a
long lapse of time — in this case, natural selection has succeeded in giving a
fixed character to the organ, in however extraordinary a manner it may
have been developed. Species inheriting nearly the same constitution from
a common parent, and exposed to similar influences, naturally tend to
present analogous variations, or these same species may occasionally revert
to some of the characters of their ancient progenitors. Although new and
important modifications may not arise from reversion and analogous varia-
tion, such modifications will add to the beautiful and harmonious diversity
of nature.

Whatever the cause may be of each slight difference between the off-
spring and their parents — and a cause for each must exist — we have reason
to believe that it is the steady accumulation of beneficial differences which
has given rise to all the more important modifications of structure in rela-
tion to the habits of each species.


Difficulties of the Theory

Difficulties of the Theory of Descent with Modification — Absence or Rarity of Transi-
tional Varieties^Transitions in Habits of Life — Diversified Habits in the Same
Species — Species with Habits Widely Different from Those of Their Allies —
Organs of Extreme Perfection — Modes of Transition — Cases of Difficulty —
Natura Non Facit Saltum — Organs of Small Importance — Organs not in all
Cases Absolutely Perfect — The Law of Unity of Type and of the Conditions
of Existence embraced by the Theory of Natural Selection.

Long before the reader has arrived at this part of my work, a crowd of
difficulties will have occurred to him. Some of them are so serious that to
this day I can hardly reflect on them without being in some degree
staggered; but, to the best of my judgment, the greater number are only
apparent, and those that are real are not, I think, fatal to the theory.

These difficulties and objections may be classed under the following
heads: First, why, if species have descended from other species by fine
gradations, do we not everywhere see innumerable transitional forms?
Why is not all nature in confusion, instead of the species being, as we see
them, well defined?

Secondly, is it possible that an animal having, for instance, the structure
and habits of a bat, could have been formed by the modification of some
other animal with widely different habits and structure? Can we believe
that natural selection could produce, on the one hand, an organ of trifling
importance, such as the tail of a giraffe, which serves as a fly-flapper, and,
on the other hand, an organ so wonderful as the eye?

Thirdly, can instincts be acquired and modified through natural selec-
tion? What shall we say to the instinct which leads the bee to make cells,
and which has practically anticipated the discoveries of profound mathe-

Fourthly, how can we account for species, when crossed, being sterile and
producing sterile offspring, whereas, when varieties are crossed, their
fertility is unimpaired?

The two first heads here will be discussed; some miscellaneous objections in
the following chapter; Instinct and Hybridism in the two succeeding chapters.


As natural selection acts solely by the preservation of profitable modifica-
tions, each new form will tend in a fully stocked country to take the place
of, and finally to exterminate, its own less improved parent-form and other
less-favored forms with which it comes into competition. Thus extinction
and natural selection go hand in hand. Hence, if we look at each species
as descended from some unknown form, both the parent and all the transi-
tional varieties will generally have been exterminated by the very process
of the formation and perfection of the new form.



But as by this theory innumerable transitional forms must have existed,
why do we not find them imbedded in countless numbers in the crust of
the earth^ It will be more convenient to discuss this question m the chapter
on the Imperfection of the Geological Record; and I will here only state
that I beheve the answer mainly lies in the record bemg incom^rably_^ .Less
perfect than is gen^Slysuppo^^

Online LibraryCharles DarwinThe origin of species → online text (page 15 of 50)