Charles Darwin.

The origin of species online

. (page 2 of 50)
Online LibraryCharles DarwinThe origin of species → online text (page 2 of 50)
Font size
QR-code for this ebook


and which cannot be accounted for by inheritance from either parent or from
some more remote ancestor. Even strongly marked differences occasionally
appear in the young of the same litter, and in seedlings from the same seed-
capsule. At long intervals of time, out of millions of individuals reared in the
same country and fed on nearly the same food, deviations of structure so
strongly pronounced as to deserve to be called monstrosities arise; but mon-
strosities cannot be separated by any distinct line from slighter variations. All
such changes of structure, whether extremely slight or strongly marked, which
appear among many individuals living together, may be considered as the in-
definite effects of the conditions of life on each individual organism, in nearly
the same manner as the chill affects different men in an indefinite manner,
according to their state of body or constitution, causing coughs or colds, rheu-
matism, or inflammation of various organs.

With respect to what I have called the indirect action of changed condi-
tions, namely, through the reproductive system being affected, we may infer
that variability is thus induced, partly from the fact of this system being ex-
tremely sensitive to any change in the conditions, and partly from the similar-
ity, as Kolreuter and others have remarked, between the variability which
follows from the crossing of distinct species, and that which may be observed
with plants and animals when reared under new or unnatural conditions.
Many facts clearly show how eminently susceptible the reproductive system is
to very slight changes in the surrounding conditions. Nothing is more easy
than to tame an animal, and few things more difficult than to get it to breed
freely under confinement, even when the male and female unite. How many
animals there are which will not breed, though kept in an almost free state
in their native country! This is generally, but erroneously, attributed to viti-
ated instincts. Many cultivated plants display the utmost vigor, and yet rarely
or never seed. In some few cases it has been discovered that a very trifling
change, such as a little more or less water at some particular period of growth,
will determine whether or not a plant will produce seeds. I cannot here give
the details which I have collected and elsewhere published on this curious sub-
ject ; but to show how singular the laws are which determine the reproduction
of animals under confinement, I may mention that carnivorous animals, even



6 THE ORIGIN OF SPECIES

from the tropics, breed in this country pretty freely under confinement, with
the exception of the plantigrades or bear family, which seldom produce
young; whereas carnivorous birds, with the rarest exceptions, hardly ever lay
fertile eggs. Many exotic plants have pollen utterly worthless, in the same con-
dition as in the most sterile hybrids. When, on the one hand, we see domesti-
cated animals and plants, though often weak and sickly, breeding freely under
confinement; and when, on the other hand, we see individuals, though taken
young from a state of nature perfectly tamed, long-lived and healthy (of
which I could give numerous instances) , yet having their reproductive system
so seriously aff"ected by unperceived causes as to fail to act, we need not be
surprised at this system, when it does act under confinement, acting irregu-
larly, and producing offspring somewhat unlike their parents. I may add that
as some organisms breed freely under the most unnatural conditions — for in-
stance, rabbits and ferrets kept in hutches — showing that their reproductive
organs are not easily affected; so will some animals and plants withstand do-
mestication or cultivation, and vary very slightly — perhaps hardly more than
in a state of nature.

Some naturalists have maintained that all variations are connected with
the act of sexual reproduction; but this is certainly an error; for I have given
in another work a long list of "sporting plants," as they are called by garden-
ers ; that is, of plants which have suddenly produced a single bud with a new
and sometimes widely different character from that of the other buds on the
same plant. These bud variations, as they may be named, can be propagated
by grafts, offsets, etc., and sometimes by seed. They occur rarely under nature,
but are far from rare under culture. As a single bud out of many thousands
produced year after year on the same tree under uniform conditions, has been
known suddenly to assume a new character; and as buds on distinct trees,
growing under different conditions, have sometimes yielded nearly the same
variety — for instance, buds on peach-trees producing nectarines, and buds on
common roses producing moss-roses — we clearly see that the nature of the
conditions is of subordinate importance in comparison with the nature of the
organism in determining each particular form of variation; perhaps of not
more importance than the nature of the spark, by which a mass of com-
bustible matter is ignited, has in determining the nature of the flames.

EFFECTS OF HABIT AND OF THE USE OR DISUSE OF PARTS;
CORRELATED VARIATION; INHERITANCE

Changed habits produce an inherited effect, as in the period of the flower-
ing of plants when transported from one climate to another. With animals
the increased use or disuse of parts has had a more marked influence; thus I
find in the domestic duck that the bones of the wing weigh less and the bones
of the leg more, in proportion to the whole skeleton, than do the same bones
in the wild duck: and this change may be safely attributed to the domestic
duck flying much less, and walking more, than its wild parents. The great
and inherited development of the udders in cows and goats in countries where



VARIATION UNDER DOMESTICATION 7

they are habitually milked, in comparison with these organs in other coun-
tries, is probably another instance of the effects of use. Not one of our domes-
tic animals can be named which has not in some country drooping ears ; and
the view which has been suggested that the drooping is due to disuse of the
muscles of the ear, from the animals being seldom much alarmed, seems ,
\ probable. - -J

Many laws regulate variation, some few of which can be dimly seen, and
will hereafter be briefly discussed. I will here only allude to what may be
called correlated variation. Important changes in the embryo or larva will
probably entail changes in the mature animal. In monstrosities, the correla-
tions between quite distinct parts are very curious; and many instances are
given in Isidore Geoffroy Saint-Hilaire's great work on this subject. Breeders
beUeve that long limbs are almost always accompanied by an elongated head.
Some instances of correlation are quite whimsical; thus cats which are en-
tirely white and have blue eyes are generally deaf; but it has been lately stated
by Mr. Tait that this is confined to the males. Color and constitutional pecu-
liarities go together, of which many remarkable cases could be given among
animals and plants. From facts collected by Heusinger, it appears that white
sheep and pigs are injured by certain plants, while dark-colored individuals
escape : Professor Wyman has recently communicated to me a good illustra-
tion of this fact; on asking some farmers in Virginia how it was that all their
pigs were black, they informed him that the pigs ate the paint-root (Lach-
nanthes), which colored their bones pink, and which caused the hoofs of all
but the black varieties to drop off: and one of the "Crackers" (i.e., Virginia
squatters) added, "We select the black members of a Utter for raising, as they-
alone have a good chance of living." Hairless dogs have imperfect teeth; long^
haired and coarse-haired animals are apt to have, as is asserted, long or many
horns ; pigeons with feathered feet have skin between their outer toes ; pigeons-
with short beaks have small feet, and those with long beaks large feet. Hence
if man goes on selecting, and thus augmenting, any peculiarity, he will almost
certainly modify unintentionally other parts of the structure, owing to the
mysterious laws of correlation.

The results of the various, unknown, or but dimly understood laws of vari'
ation are infinitely complex and diversified. It is well worth while carefully to
study the several treatises on some of our old cultivated plants, as on the hya-
cinth, potato, even the dahlia, etc. ; and it is really surprising to note the end-
less points of structure and constitution in which the varieties and sub-vari-
eties differ slightly from each other. The whole organization seems to have
become plastic, and departs in a slight degree from that of the parental type.

Any variation which is not inherited is unimportant for us. But the num-
ber and diversity of inheritable deviations of structure, both those of slight
and those of considerable physiological importance, are endless. Dr. Prosper
Lucas' treatise, in two large volumes, is the fullest and the best on this sub-
ject. No breeder doubts how strong is the tendency to inheritance; that like
produces like, is his fundamental belief; doubts have been thrown on this
principle only by theoretical writers. When any deviation of structure often



8 THE ORIGIN OF SPECIES

appears, and we see it in the father and child, we cannot tell whether it may
not be due to the same cause having acted on both; but when among indi-
viduals, apparently exposed to the same conditions, any very rare deviation,
due to some extraordinary combination of circumstances, appears in the par-
ent — say, once among several million individuals — and it reappears in the
child, the mere doctrine of chances almost compels us to attribute its reap-
pearance to inheritance. Every one must have heard of cases of albinism,
prickly skin, hairy bodies, etc., appearing in several members of the same
family. If strange and rare deviations of structure are really inherited, less
strange and commoner deviations may be freely admitted to be inheritable.
Perhaps the correct way of viewing the whole subject would be, to look at the
inheritance of every character whatever as the rule, and non-inheritance as
the anomaly.

The laws governing inheritance are for the most part unknown. No one can
say why the same peculiarity in different individuals of the same species, or in
different species, is sometimes inherited and sometimes not so; why the child
often reverts in certain characteristics to its grandfather or grandmother or
more remote ancestor; why a peculiarity is often transmitted from one sex
to both sexes, or to one sex alone, more commonly but not exclusively to the
like sex. It is a fact of some importance to us, that peculiarities appearing in
the males of our domestic breeds are often transmitted, either exclusively or
in a much greater degree, to the males alone. A much more important rule,
which I think may be trusted, is that, at whatever period of life a peculiarity
first appears, it tends to reappear in the offspring at a corresponding age,
though sometimes earlier. In many cases this could not be otherwise : thus the
inherited peculiarities in the horns of cattle could appear only in the offspring
when nearly mature ; peculiarities in the silkworm are known to appear at the
corresponding caterpillar or cocoon stage. But hereditary diseases and some
other facts make me believe that the rule has a wider extension, and that,
when there is no apparent reason why a peculiarity should appear at any
particular age, yet that it does tend to appear in the offspring at the same
period at which it first appeared in the parent. I believe this rule to be of the
highest importance in explaining the laws of embryology. These remarks are
of course confined to the first appearance of the peculiarity, and not to the
primary cause which may have acted on the ovules or on the male element;
in nearly the same manner as the increased length of the horns in the off-
spring from a short-horned cow by a long-horned bull, though appearing
late in life, is clearly due to the male element.

Having alluded to the subject of reversion, I may here refer to a statement
often made by naturalists — namely, that our domestic varieties, when run
wild, gradually but invariably revert in character to their aboriginal stock.
Hence it has been argued that no deductions can be drawn from domestic
races to species in a state of nature. I have in vain endeavored to discover
on what decisive facts the above statement has so often and so boldly been
made. There would be great difficulty in proving its truth: we may safely
conclude that very many of the most strongly marked domestic varieties could



VARIATION UNDER DOMESTICATION 9

not possibly live in a wild state. In many cases we do not know what the
aboriginal stock was, and so could not tell whether or not nearly perfect re-
version had ensued. It would be necessary, in order to prevent the effects of
intercrossing, that only a single variety should have been turned loose in its
new home. Nevertheless, as our varieties certainly do occasionally revert in
some of their characters to ancestral forms, it seems to me not improbable
that if we could succeed in naturalizing, or were to cultivate, during many
generations, the several races, for instance, of the cabbage, in very poor soil —
in which case, however, some effect would have to be attributed to the defi-
nite action of the poor soil — that they would, to a large extent, or even wholly,
revert to the wild aboriginal stock. Whether or not the experiment would suc-
ceed is not of great importance for our line of argument; for by the experi-
ment itself the conditions of life are changed. If it could be shown that our
domestic varieties manifested a strong tendency to reversion — that is, to lose
their acquired characters, while kept under the same conditions and while
kept in a considerable body, so that free intercrossing might check, by blend-
ing together, any slight deviations in their structure, in such case, I grant that
we could deduce nothing from domestic varieties in regard to species. But
there is not a shadow of evidence in favor of this view : to assert that we could
not breed our cart and race horses, long and short horned cattle, and poultry
of various breeds, and esculent vegetables, for an unlimited number of gen-
erations, would be opposed to all experience.

CHARACTER OF DOMESTIC VARIETIES; DIFFICULTY OF DISTINGUISHING BETWEEN
VARIETIES AND SPECIES; ORIGIN OF DOMESTIC VARIETIES FROM ONE OR MORE
SPECIES

When we look to the hereditary varieties or races of our domestic animals
and plants, and compare them with closely allied species, we generally per-
ceive in each domestic race, as already remarked, less uniformity of charac-
ter than in true species. Domestic races often have a somewhat monstrous
character; by which I mean, that, although differing from each other and
from other species of the same genus, in several trifling respects, they often
differ in an extreme degree in some one part, both when compared one with
another, and more especially when compared with the species under nature
to which they are nearest allied. With these exceptions (and with that of the
perfect fertility of varieties when crossed — a subject hereafter to be discussed) ,
domestic races of the same species differ from each other in the same manner
as do the closely allied species of the same genus in a state of nature, but the
differences in most cases are less in degree. This must be admitted as true, for
the domestic races of many animals and plants have been ranked by some
competent judges as the descendants of aboriginally distinct species, and by
other competent judges as mere varieties. If any well-marked distinction
existed between a domestic race and a species, this source of doubt would not
so perpetually recur. It has often been stated that domestic races do not differ
from each other in characters of generic value. It can be shown that this



10 THE ORIGIN OF SPECIES

statement is not correct; but naturalists differ much in determining what
characters are of generic value ; all such valuations being at present empirical.
When it is explained how genera originate under nature, it will be seen that
we have no right to expect often to find a generic amount of difference in
our domesticated races.

In attempting to estimate the amount of structural difference between
allied domestic races, we are soon involved in doubt, from not knowing
whether they are descended from one or several parent species. This point, if
it could be cleared up, would be interesting; if, for instance, it could be
shown that the greyhound, bloodhound, terrier, spaniel, and bull-dog, which
we all know propagate their kind truly, were the offspring of any single spe-
cies, then such facts would have great weight in making us doubt about the
immutability of the many closely allied natural species — for instance, of the
many foxes — inhabiting the different quarters of the world. I do not believe,
as we shall presently see, that the whole amount of difference between the
several breeds of the dog has been produced under domestication; I believe
that a small part of the difference is due to their being descended from dis-
tinct species. In the case of strongly marked races of some other domesticated
species, there is presumptive or even strong evidence that all are descended
from a single wild stock.

It has often been assumed that man has chosen for domestication animals
and plants having an extraordinary inherent tendency to vary, and likewise
to withstand diverse climates. I do not dispute that these capacities have
added largely to the value of most of our domesticated productions ; but how
could a savage possibly know, when he first tamed an animal, whether it
would vary in succeeding generations, and whether it would endure other cli-
mates? Has the little variability of the ass and goose, or the small power of
endurance of warmth by the reindeer, or of cold by the common camel, pre-
vented their domestication? I cannot doubt that if other animals and plants,
equal in number to our domesticated productions, and belonging to equally
diverse classes and countries, were taken from a state of nature, and could be
made to breed for an equal number of generations under domestication, they
would on an average vary as largely as the parent species of our existing do-
mesticated productions have varied.

In the case of most of our anciently domesticated animals and plants, it is
not possible to come to any definite conclusion, whether they are descended
from one or several wild species. The argument mainly relied on by those
who believe in the multiple origin of our domestic animals is, that we find in
the most ancient times, on the monuments of Egypt, and in the lake-habita-
tions of Switzerland, much diversity in the breeds; and that some of these
ancient breeds closely resemble, or are even identical with, those still
existing. But this only throws far backward the history of civilization, and
shows that animals were domesticated at a much earlier period than has
hitherto been supposed. The lake-inhabitants of Switzerland cultivated
several kinds of wheat and barley, the pea, the poppy for oil, and flax; and
they possessed several domesticated animals. They also carried on commerce



VARIATION UNDER DOMESTICATION 11

with other nations. All this clearly shows, as Heer has remarked, that they
had at this early age progressed considerably in civilization; and this again
implies a long continued previous period of less advanced civilization, during
which the domesticated animals, kept by different tribes in different districts,
might have varied and given rise to distinct races. Since the discovery of
flint tools in the superficial formations of many parts of the world, all
geologists believe that barbarian men existed at an enormously remote
period; and we know that at the present day there is hardly a tribe so
barbarous as not to have domesticated at least the dog.

The origin of most of our domestic animals will probably forever remain
vague. But I may here state that, looking to the domestic dogs of the whole
world, I have, after a laborious collection of all known facts, come to the
conclusion that several wild species of Canidae have been tamed, and that
their blood, in some cases mingled together, flows in the veins of our
domestic breeds. In regard to sheep and goats I can form no decided opinion.
From facts communicated to me by Mr. Blyth, on the habits, voice, con-
stitution, and structure of the humped Indian cattle, it is almost certain
that they are descended from a different aboriginal stock from our European
cattle; and some competent judges believe that these latter have had two
or three wild progenitors, whether or not these deserve to be called species.
This conclusion, as well as that of the specific distinction between the
humped and common cattle, may, indeed, be looked upon as established by
the admirable researches of Professor Riitimeyer. With respect to horses,
from reasons which I cannot here give, I am doubtfully inclined to believe,
in opposition to several authors, that all the races belong to the same species.
Having kept nearly all the English breeds of the fowl alive, having bred and
crossed them, and examined their skeletons, it appears to me almost certain
that all are the descendants of the wild Indian fowl, Gallus bankiva; and
this is the conclusion of Mr. Blyth, and of others who have studied this bird
in India. In regard to ducks and rabbits, some breeds of which differ much
from each other, the evidence is clear that they are all descended from the
common duck and wild rabbit.

The doctrine of the origin of our several domestic races from several
aboriginal stocks, has been carried to an absurd extreme by some authors.
They believe that every race which breeds true, let the distinctive characters
be ever so slight, has had its wild prototype. At this rate there must have
existed at least a score of species of wild cattle, as many sheep, and several
goats, in Europe alone, and several even within Great Britain. One author
believes that there formerly existed eleven wild species of sheep peculiar to
Great Britain! When we bear in mind that Britain has now not one peculiar
mammal, and France but few distinct from those of Germany, and so with
Hungary, Spain, etc., but that each of these kingdoms possesses several
peculiar breeds of cattle, sheep, etc., we must admit that many domestic
breeds must have originated in Europe; for whence otherwise could they
have been derived? So it is in India. Even in the case of the breeds of the
domestic dog throughout the world, which I admit are descended from



12 THE ORIGIN OF SPECIES

several wild species, it cannot be doubted that there has been an immense
amount of inherited variation; for who will believe that animals closely
resembling the Italian greyhound, the bloodhound, the bull-dog, pug-dog,
or Blenheim spaniel, etc. — so unlike all wild Canidae — ever existed in a state
of nature? It has often been loosely said that all our races of dogs have been
produced by the crossing of a few aboriginal species; but by crossing we can
only get forms in some degree intermediate between their parents; and if
we account for our several domestic races by this process, we must admit
the former existence of the most extreme forms, as the Italian greyhound,
bloodhound, bull-dog, etc., in the wild state. Moreover, the possibility of
making distinct races by crossing has been greatly exaggerated. Many cases
are on record showing that a race may be modified by occasional crosses if
aided by the careful selection of the individuals which present the desired
character; but to obtain a race intermediate between two quite distinct races
would be very difficult. Sir J. Sebright expressly experimented with this
object and failed. The offspring from the first cross between two pure breeds
is tolerably and sometimes (as I have found with pigeons) quite uniform in
character, and every thing seems simple enough; but when these mongrels
are crossed one with another for several generations, hardly two of them are
alike, and then the difficulty of the task becomes manifest.

BREEDS OF THE DOMESTIC PIGEON, THEIR DIFFERENCES AND ORIGIN

Believing that it is always best to study some special group, I have, after
deliberation, taken up domestic pigeons. I have kept every breed which I
could purchase or obtain, and have been most kindly favored with skins



Online LibraryCharles DarwinThe origin of species → online text (page 2 of 50)