low in the scale of organization, possess some degree of intelligence.
This will strike every one as very improbable; but it may be doubted
whether we know enough about the nervous system of the lower animals to
justify our natural distrust of such a conclusion. With respect to the
small size of the cerebral ganglia, we should remember what a mass of
inherited knowledge, with some power of adapting means to an end, is
crowded into the minute brain of a worker-ant.
III.
THE LAWS OF VARIABILITY WITH RESPECT TO ANIMALS AND PLANTS.
[The Variation
of Animals and
Plants under
Domestication,
vol. i,
page 3.]
I shall in this volume treat, as fully as my materials permit, the
whole subject of variation under domestication. We may thus hope to
obtain some light, little though it be, on the causes of variability,
on the laws which govern it--such as the direct action of climate and
food, the effects of use and disuse, and of correlation of growth--and
on the amount of change to which domesticated organisms are liable.
* * * * *
Although man does not cause variability and can not even prevent it,
he can select, preserve, and accumulate the variations given to him
by the hand of Nature almost in any way which he chooses; and thus he
can certainly produce a great result. Selection may be followed either
methodically and intentionally, or unconsciously and unintentionally.
Man may select and preserve each successive variation, with the
distinct intention of improving and altering a breed, in accordance
with a preconceived idea; and by thus adding up variations, often so
slight as to be imperceptible by an uneducated eye, he has effected
wonderful changes and improvements. It can, also, be clearly shown
that man, without any intention or thought of improving the breed,
by preserving in each successive generation the individuals which he
prizes most, and by destroying the worthless individuals, slowly,
though surely, induces great changes. As the will of man thus comes
into play, we can understand how it is that domesticated breeds show
adaptation to his wants and pleasures. We can further understand how it
is that domestic races of animals and cultivated races of plants often
exhibit an abnormal character, as compared with natural species; for
they have been modified not for their own benefit, but for that of man.
INHERITED EFFECT OF CHANGED HABITS.
[Origin of
Species,
page 5.]
When we compare the individuals of the same variety or subvariety of
our older cultivated plants and animals, one of the first points which
strikes us is, that they generally differ more from each other than do
the individuals of any one species or variety in a state of nature. And
if we reflect on the vast diversity of the plants and animals which
have been cultivated, and which have varied during all ages under
the most different climates and treatment, we are driven to conclude
that this great variability is due to our domestic productions having
been raised under conditions of life not so uniform as, and somewhat
different from, those to which the parent species had been exposed
under nature.
* * * * *
[Page 8.]
Changed habits produce an inherited effect, as in the period of the
flowering 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 they are habitually
milked, in comparison with these organs in other countries, is probably
another instance of the effects of use. Not one of our domestic 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 the disease of the
muscles of the ear, from the animals being seldom much alarmed, seems
probable.
* * * * *
[Page 9.]
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 illustration
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 (_Lachnanthes_), 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 litter 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.
EFFECTS OF THE USE AND DISUSE OF PARTS.
[Origin of
Species,
page 108.]
From the facts alluded to in the first chapter, I think there can be no
doubt that use in our domestic animals has strengthened and enlarged
certain parts, and disuse diminished them, and that such modifications
are inherited. Under free nature we have no standard of comparison by
which to judge of the effects of long-continued use or disuse, for we
know not the parent forms; but many animals possess structures which
can be best explained by the effects of disuse. As Professor Owen has
remarked, there is no greater anomaly in nature than a bird that can
not fly; yet there are several in this state. The logger-headed duck of
South America can only flap along the surface of the water, and has its
wings in nearly the same condition as the domestic Aylesbury duck: it
is a remarkable fact that the young birds, according to Mr. Cunningham,
can fly, while the adults have lost this power. As the larger
ground-feeding birds seldom take flight, except to escape danger, it
is probable that the nearly wingless condition of several birds, now
inhabiting or which lately inhabited several oceanic islands, tenanted
by no beast of prey, has been caused by disuse. The ostrich, indeed,
inhabits continents, and is exposed to danger from which it can not
escape by flight, but it can defend itself by kicking its enemies as
efficiently as many quadrupeds. We may believe that the progenitor
of the ostrich genus had habits like those of the bustard, and that,
as the size and weight of its body were increased during successive
generations, its legs were used more, and its wings less, until they
became incapable of flight.
* * * * *
[Page 109.]
The insects in Madeira which are not ground-feeders, and which, as
certain flower-feeding _Coleoptera_ and _Lepidoptera_, must habitually
use their wings to gain their subsistence, have, as Mr. Wollaston
suspects, their wings not at all reduced, but even enlarged. This is
quite compatible with the action of natural selection. For, when a new
insect first arrived on the island, the tendency of natural selection
to enlarge or to reduce the wings would depend on whether a greater
number of individuals were saved by successfully battling with the
winds, or by giving up the attempt and rarely or never flying. As with
mariners shipwrecked near a coast, it would have been better for the
good swimmers if they had been able to swim still farther, whereas it
would have been better for the bad swimmers if they had not been able
to swim at all and had stuck to the wreck.
The eyes of moles and of some burrowing rodents are rudimentary in
size, and in some cases are quite covered by skin and fur. This state
of the eyes is probably due to gradual reduction from disuse, but
aided, perhaps, by natural selection. In South America a burrowing
rodent--the tuco-tuco, or ctenomys--is even more subterranean in its
habits than the mole; and I was assured by a Spaniard, who had often
caught them, that they were frequently blind. One which I kept alive
was certainly in this condition, the cause, as appeared on dissection,
having been inflammation of the nictitating membrane. As frequent
inflammation of the eyes must be injurious to any animal, and as eyes
are certainly not necessary to animals having subterranean habits, a
reduction in their size, with the adhesion of the eyelids and growth of
fur over them, might in such case be an advantage; and, if so, natural
selection would aid the effects of disuse.
VAGUE ORIGIN OF OUR DOMESTIC ANIMALS.
[Origin of
Species,
page 13.]
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-habitations 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 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 man
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.
* * * * *
[Page 12.]
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 species. 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 different quarters of the world.
DESCENT OF THE DOMESTIC PIGEON.
[Origin of
Species,
page 17.]
Great as are the differences between the breeds of the pigeon, I am
fully convinced that the common opinion of naturalists is correct,
namely, that all are descended from the rock-pigeon (_Columba livia_),
including under this term several geographical races or sub-species,
which differ from each other in the most trifling respects. As several
of the reasons which have led me to this belief are in some degree
applicable in other cases, I will here briefly give them. If the
several breeds are not varieties, and have not proceeded from the
rock-pigeon, they must have descended from at least seven or eight
aboriginal stocks; for it is impossible to make the present domestic
breeds by the crossing of any lesser number: how, for instance,
could a pouter be produced by crossing two breeds unless one of the
parent-stocks possessed the characteristic enormous crop? The supposed
aboriginal stocks must all have been rock-pigeons--that is, they did
not breed or willingly perch on trees. But besides _C. livia_, with
its geographical sub-species, only two or three other species of
rock-pigeons are known, and these have not any of the characters of the
domestic breeds. Hence the supposed aboriginal stocks must either still
exist in the countries where they were originally domesticated, and yet
be unknown to ornithologists--and this, considering their size, habits,
and remarkable characters, seems improbable--or they must have become
extinct in the wild state. But birds breeding on precipices, and good
fliers, are unlikely to be exterminated; and the common rock-pigeon,
which has the same habits with the domestic breeds, has not been
exterminated even on several of the smaller British islets, or on the
shores of the Mediterranean. Hence the supposed extermination of so
many species having similar habits with the rock-pigeon seems a very
rash assumption. Moreover, the several above-named domesticated breeds
have been transported to all parts of the world, and therefore some of
them must have been carried back again into their native country; but
not one has become wild or feral, though the dovecot-pigeon, which is
the rock-pigeon in a very slightly altered state, has become feral in
several places. Again, all recent experience shows that it is difficult
to get wild animals to breed freely under domestication; yet, on the
hypothesis of the multiple origin of our pigeons, it must be assumed
that at least seven or eight species were so thoroughly domesticated
in ancient times by half-civilized man as to be quite prolific under
confinement.
An argument of great weight, and applicable in several other cases,
is, that the above-specified breeds, though agreeing generally with
the wild rock-pigeon in constitution, habits, voice, coloring, and
in most parts of their structure, yet are certainly highly abnormal
in other parts; we may look in vain through the whole great family
of _Columbidæ_ for a beak like that of the English carrier, or
that of the short-faced tumbler, or barb; for reversed feathers
like those of the Jacobin; for a crop like that of the pouter; for
tail-feathers like those of the fantail. Hence it must be assumed not
only that half-civilized man succeeded in thoroughly domesticating
several species, but that he intentionally or by chance picked out
extraordinarily abnormal species; and, further, that these very species
have since all become extinct or unknown. So many strange contingencies
are improbable in the highest degree.
ORIGIN OF THE DOG.
[Animals and
Plants under
Domestication,
vol. i,
page 15.]
The first and chief point of interest in this chapter is, whether
the numerous domesticated varieties of the dog have descended from a
single wild species, or from several. Some authors believe that all
have descended from the wolf, or from the jackal, or from an unknown
and extinct species. Others again believe, and this of late has been
the favorite tenet, that they have descended from several species,
extinct and recent, more or less commingled together. We shall probably
never be able to ascertain their origin with certainty. Paleontology
does not throw much light on the question, owing, on the one hand, to
the close similarity of the skulls of extinct as well as living wolves
and jackals, and owing, on the other hand, to the great dissimilarity
of the skulls of the several breeds of the domestic dogs. It seems,
however, that remains have been found in the later tertiary deposits
more like those of a large dog than of a wolf, which favors the belief
of De Blainville that our dogs are the descendants of a single extinct
species. On the other hand, some authors go so far as to assert that
every chief domestic breed must have had its wild prototype. This
latter view is extremely improbable: it allows nothing for variation;
it passes over the almost monstrous character of some of the breeds;
and it almost necessarily assumes that a large number of species have
become extinct since man domesticated the dog; whereas we plainly see
that wild members of the dog-family are extirpated by human agency with
much difficulty; even so recently as 1710 the wolf existed in so small
an island as Ireland.
* * * * *
[Page 18.]
At a period between four and five thousand years ago, various
breeds--viz., pariah dogs, greyhounds, common hounds, mastiffs,
house-dogs, lap-dogs, and turnspits--existed, more or less closely
resembling our present breeds. But there is not sufficient evidence
that any of these ancient dogs belonged to the same identical
sub-varieties with our present dogs. As long as man was believed to
have existed on this earth only about six thousand years, this fact of
the great diversity of the breeds at so early a period was an argument
of much weight that they had proceeded from several wild sources, for
there would not have been sufficient time for their divergence and
modification. But now that we know, from the discovery of flint tools
imbedded with the remains of extinct animals, in districts which have
since undergone great geographical changes, that man has existed for an
incomparably longer period, and bearing in mind that the most barbarous
nations possess domestic dogs, the argument from insufficient time
falls away greatly in value.
[Page 26.]
From this resemblance of the half-domesticated dogs in several
countries to the wild species still living there--from the facility
with which they can often be crossed together--from even half-tamed
animals being so much valued by savages--and from the other
circumstances previously remarked on which favor their domestication,
it is highly probable that the domestic dogs of the world are descended
from two well-defined species of wolf (viz., _C. lupus_ and _C.
latrans_), and from two or three other doubtful species (namely, the
European, Indian, and North African wolves); from at least one or two
South American canine species; from several races or species of jackal;
and perhaps from one or more extinct species.
ORIGIN OF THE HORSE.
[Animals and
Plants under
Domestication,
vol. i,
page 51.]
The history of the horse is lost in antiquity. Remains of this
animal in a domesticated condition have been found in the Swiss
lake-dwellings, belonging to the Neolithic period. At the present
time the number of breeds is great, as may be seen by consulting any
treatise on the horse. Looking only to the native ponies of Great
Britain, those of the Shetland Isles, Wales, the New Forest, and
Devonshire are distinguishable; and so it is, among other instances,
with each separate island in the great Malay Archipelago. Some of the
breeds present great differences in size, shape of ears, length of
mane, proportions of the body, form of the withers and hind-quarters,
and especially in the head. Compare the race-horse, dray-horse, and a
Shetland pony in size, configuration, and disposition; and see how much
greater the difference is than between the seven or eight other living
species of the genus _Equus_.
* * * * *
[Page 52.]
Horses have often been observed, according to M. Gaudry, to possess
a trapezium and a rudiment of a fifth metacarpal bone, so that “one
sees appearing by monstrosity, in the foot of the horse, structures
which normally exist in the foot of the hipparion”--an allied and
extinct animal. In various countries horn-like projections have been
observed on the frontal bones of the horse: in one case described by
Mr. Percival they arose about two inches above the orbital processes,
and were “very like those in a calf from five to six months old,” being
from half to three quarters of an inch in length.
CAUSES OF MODIFICATIONS IN THE HORSE.
[Page 54.]
With respect to the causes of the modifications which horses have
undergone, the conditions of life seem to produce a considerable direct
effect. Mr. D. Forbes, who has had excellent opportunities of comparing
the horses of Spain with those of South America, informs me that the
horses of Chili, which have lived under nearly the same conditions as
their progenitors in Andalusia, remain unaltered, while the Pampas
horses and the Puno ponies are considerably modified. There can be
no doubt that horses become greatly reduced in size and altered in
appearance by living on mountains and islands; and this apparently is
due to want of nutritious or varied food. Every one knows how small and
rugged the ponies are on the northern islands and on the mountains of
Europe. Corsica and Sardinia have their native ponies; and there were,
or still are, on some islands on the coast of Virginia, ponies like
those of the Shetland Islands, which are believed to have originated
through exposure to unfavorable conditions. The Puno ponies, which
inhabit the lofty regions of the Cordillera, are, as I hear from
Mr. D. Forbes, strange little creatures, very unlike their Spanish
progenitors. Farther south, in the Falkland Islands, the offspring of
the horses imported in 1764 have already so much deteriorated in size
and strength, that they are unfitted for catching wild cattle with the
lasso; so that fresh horses have to be brought for this purpose from
La Plata at a great expense. The reduced size of the horses bred on
both southern and northern islands, and on several mountain-chains,
can hardly have been caused by the cold, as a similar reduction has
occurred on the Virginian and Mediterranean islands.
* * * * *
[Page 56.]
It is scarcely possible to doubt that the long-continued selection of
qualities serviceable to man has been the chief agent in the formation
of the several breeds of the horse. Look at a dray-horse, and see how
well adapted he is to draw heavy weights, and how unlike in appearance
to any allied wild animal. The English race-horse is known to be
derived from the commingled blood of Arabs, Turks, and Barbs; but
selection, which was carried on during very early times in England,
together with training, have made him a very different animal from his
parent stocks.
“MAKING THE WORKS OF GOD A MERE MOCKERY.”
[Origin of
Species,
page 130.]