VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED PLANTS
The facts of variation and artificial selection--Proofs of the
generality of variation--Variations of apples and
melons--Variations of flowers--Variations of domestic
animals--Domestic pigeons--Acclimatisation--Circumstances
favourable to selection by man--Conditions favourable to
variation--Concluding remarks.
Having so fully discussed variation under nature it will be unnecessary
to devote so much space to domesticated animals and cultivated plants,
especially as Mr. Darwin has published two remarkable volumes on the
subject where those who desire it may obtain ample information. A
general sketch of the more important facts will, however, be given, for
the purpose of showing how closely they correspond with those described
in the preceding chapter, and also to point out the general principles
which they illustrate. It will also be necessary to explain how these
variations have been increased and accumulated by artificial selection,
since we are thereby better enabled to understand the action of natural
selection, to be discussed in the succeeding chapter.
_The facts of Variation and Artificial Selection._
Every one knows that in each litter of kittens or of puppies no two are
alike. Even in the case in which several are exactly alike in colours,
other differences are always perceptible to those who observe them
closely. They will differ in size, in the proportions of their bodies
and limbs, in the length or texture of their hairy covering, and notably
in their disposition. They each possess, too, an individual
countenance, almost as varied when closely studied as that of a human
being; not only can a shepherd distinguish every sheep in his flock, but
we all know that each kitten in the successive families of our old
favourite cat has a face of its own, with an expression and
individuality distinct from all its brothers and sisters. Now this
individual variability exists among all creatures whatever, which we can
closely observe, even when the two parents are very much alike and have
been matched in order to preserve some special breed. The same thing
occurs in the vegetable kingdom. All plants raised from seed differ more
or less from each other. In every bed of flowers or of vegetables we
shall find, if we look closely, that there are countless small
differences, in the size, in the mode of growth, in the shape or colour
of the leaves, in the form, colour, or markings of the flowers, or in
the size, form, colour, or flavour of the fruit. These differences are
usually small, but are yet easily seen, and in their extremes are very
considerable; and they have this important quality, that they have a
tendency to be reproduced, and thus by careful breeding any particular
variation or group of variations can be increased to an enormous
extent--apparently to any extent not incompatible with the life, growth,
and reproduction of the plant or animal.
The way this is done is by artificial selection, and it is very
important to understand this process and its results. Suppose we have a
plant with a small edible seed, and we want to increase the size of that
seed. We grow as large a quantity of it as possible, and when the crop
is ripe we carefully choose a few of the very largest seeds, or we may
by means of a sieve sort out a quantity of the largest seeds. Next year
we sow only these large seeds, taking care to give them suitable soil
and manure, and the result is found to be that the _average_ size of the
seeds is larger than in the first crop, and that the largest seeds are
now somewhat larger and more numerous. Again sowing these, we obtain a
further slight increase of size, and in a very few years we obtain a
greatly improved race, which will always produce larger seeds than the
unimproved race, even if cultivated without any special care. In this
way all our fine sorts of vegetables, fruits, and flowers have been
obtained, all our choice breeds of cattle or of poultry, our wonderful
race-horses, and our endless varieties of dogs. It is a very common but
mistaken idea that this improvement is due to crossing and feeding in
the case of animals, and to improved cultivation in the case of plants.
Crossing is occasionally used in order to obtain a combination of
qualities found in two distinct breeds, and also because it is found to
increase the constitutional vigour; but every breed possessing any
exceptional quality is the result of the selection of variations
occurring year after year and accumulated in the manner just described.
Purity of breed, with repeated selection of the best varieties of that
breed, is the foundation of all improvement in our domestic animals and
cultivated plants.
_Proofs of the Generality of Variation._
Another very common error is, that variation is the exception, and
rather a rare exception, and that it occurs only in one direction at a
time--that is, that only one or two of the numerous possible modes of
variation occur at the same time. The experience of breeders and
cultivators, however, proves that variation is the rule instead of the
exception, and that it occurs, more or less, in almost every direction.
This is shown by the fact that different species of plants and animals
have required different _kinds_ of modification to adapt them to our
use, and we have never failed to meet with variation _in that particular
direction_, so as to enable us to accumulate it and so to produce
ultimately a large amount of change in the required direction. Our
gardens furnish us with numberless examples of this property of plants.
In the cabbage and lettuce we have found variation in the size and mode
of growth of the leaf, enabling us to produce by selection the almost
innumerable varieties, some with solid heads of foliage quite unlike any
plant in a state of nature, others with curiously wrinkled leaves like
the savoy, others of a deep purple colour used for pickling. From the
very same species as the cabbage (Brassica oleracea) have arisen the
broccoli and cauliflower, in which the leaves have undergone little
alteration, while the branching heads of flowers grow into a compact
mass forming one of our most delicate vegetables. The brussels sprouts
are another form of the same plant, in which the whole mode of growth
has been altered, numerous little heads of leaves being produced on the
stem. In other varieties the ribs of the leaves are thickened so as to
become themselves a culinary vegetable; while, in the Kohlrabi, the stem
grows into a turnip-like mass just above ground. Now all these
extraordinarily distinct plants come from one original species which
still grows wild on our coasts; and it must have varied in all these
directions, otherwise variations could not have been accumulated to the
extent we now see them. The flowers and seeds of all these plants have
remained nearly stationary, because no attempt has been made to
accumulate the slight variations that no doubt occur in them.
If now we turn to another set of plants, the turnips, radishes, carrots,
and potatoes, we find that the roots or underground tubers have been
wonderfully enlarged and improved, and also altered in shape and colour,
while the stems, leaves, flowers, and fruits have remained almost
unchanged. In the various kinds of peas and beans it is the pod or fruit
and the seed that has been subjected to selection, and therefore greatly
modified; and it is here very important to notice that while all these
plants have undergone cultivation in a great variety of soils and
climates, with different manures and under different systems, yet the
flowers have remained but little altered, those of the broad bean, the
scarlet-runner, and the garden-pea, being nearly the same in all the
varieties. This shows us how little change is produced by mere
cultivation, or even by variety of soil and climate, if there is no
_selection_ to preserve and accumulate the small variations that are
continually occurring. When, however, a great amount of modification has
been effected in one country, change to another country produces a
decided effect. Thus it has been found that some of the numerous
varieties of maize produced and cultivated in the United States change
considerably, not only in their size and colour, but even in the shape
of the seed when grown for a few successive years in Germany.[31] In all
our cultivated fruit trees the fruits vary immensely in shape, size,
colour, flavour, time of ripening, and other qualities, while the leaves
and flowers usually differ so little that they are hardly
distinguishable except to a very close observer.
_Variations of Apples and of Melons._
The most remarkable varieties are afforded by the apple and the melon,
and some account of these will be given as illustrating the effects of
slight variations accumulated by selection. All our apples are known to
have descended from the common crab of our hedges (Pyrus malus), and
from this at least a thousand distinct varieties have been produced.
These differ greatly in the size and form of the fruit, in its colour,
and in the texture of the skin. They further differ in the time of
ripening, in their flavour, and in their keeping properties; but apple
trees also differ in many other ways. The foliage of the different
varieties can often be distinguished by peculiarities of form and
colour, and it varies considerably in the time of its appearance; in
some hardly a leaf appears till the tree is in full bloom, while others
produce their leaves so early as almost to hide the flowers. The flowers
differ in size and colour, and in one case in structure also, that of
the St. Valery apple having a double calyx with ten divisions, and
fourteen styles with oblique stigmas, but without stamens or corolla.
The flowers, therefore, have to be fertilised with the pollen from other
varieties in order to produce fruit. The pips or seeds differ also in
shape, size, and colour; some varieties are liable to canker more than
others, while the Winter Majetin and one or two others have the strange
constitutional peculiarity of never being attacked by the mealy bug even
when all the other trees in the same orchard are infested with it.
All the cucumbers and gourds vary immensely, but the melon (Cucumis
melo) exceeds them all. A French botanist, M. Naudin, devoted six years
to their study. He found that previous botanists had described thirty
distinct species, as they thought, which were really only varieties of
melons. They differ chiefly in their fruits, but also very much in
foliage and mode of growth. Some melons are only as large as small
plums, others weigh as much as sixty-six pounds. One variety has a
scarlet fruit. Another is not more than an inch in diameter, but
sometimes more than a yard in length, twisting about in all directions
like a serpent. Some melons are exactly like cucumbers; and an Algerian
variety, when ripe, cracks and falls to pieces, just as occurs in a
wild gourd (C. momordica).[32]
_Variations of Flowers._
Turning to flowers, we find that in the same genus as our currant and
gooseberry, which we have cultivated for their fruits, there are some
ornamental species, as the Ribes sanguinea, and in these the flowers
have been selected so as to produce deep red, pink, or white varieties.
When any particular flower becomes fashionable and is grown in large
quantities, variations are always met with sufficient to produce great
varieties of tint or marking, as shown by our roses, auriculas, and
geraniums. When varied leaves are required, it is found that a number of
plants vary sufficiently in this direction also, and we have zonal
geraniums, variegated ivies, gold and silver marked hollies, and many
others.
_Variations of Domestic Animals._
Coming now to our domesticated animals, we find still more extraordinary
cases; and it appears as if any special quality or modification in an
animal can be obtained if we only breed it in sufficient quantity, watch
carefully for the required variations, and carry on selection with
patience and skill for a sufficiently long period. Thus, in sheep we
have enormously increased the wool, and have obtained the power of
rapidly forming flesh and fat; in cows we have increased the production
of milk; in horses we have obtained strength, endurance, or speed, and
have greatly modified size, form, and colour; in poultry we have secured
various colours of plumage, increase of size, and almost perpetual
egg-laying. But it is in dogs and pigeons that the most marvellous
changes have been effected, and these require our special attention.
Our various domestic dogs are believed to have originated from several
distinct wild species, because in every part of the world the native
dogs resemble some wild dogs or wolves of the same country. Thus perhaps
several species of wolves and jackals were domesticated in very early
times, and from breeds derived from these, crossed and improved by
selection, our existing dogs have descended. But this intermixture of
distinct species will go a very little way in accounting for the
peculiarities of the different breeds of dogs, many of which are totally
unlike any wild animal. Such is the case with greyhounds, bloodhounds,
bulldogs, Blenheim spaniels, terriers, pugs, turnspits, pointers, and
many others; and these differ so greatly in size, shape, colour, and
habits, as well as in the form and proportions of all the different
parts of the body, that it seems impossible that they could have
descended from any of the known wild dogs, wolves, or allied animals,
none of which differ nearly so much in size, form, and proportions. We
have here a remarkable proof that variation is not confined to
superficial characters--to the colour, hair, or external appendages,
when we see how the entire skeletons of such forms as the greyhound and
the bulldog have been gradually changed in opposite directions till they
are both completely unlike that of any known wild animal, recent or
extinct. These changes have been the result of some thousands of years
of domestication and selection, different breeds being used and
preserved for different purposes; but some of the best breeds are known
to have been improved and perfected in modern times. About the middle of
the last century a new and improved kind of foxhound was produced; the
greyhound was also greatly improved at the end of the last century,
while the true bulldog was brought to perfection about the same period.
The Newfoundland dog has been so much changed since it was first
imported that it is now quite unlike any existing native dog in that
island.[33]
_Domestic Pigeons._
The most remarkable and instructive example of variation produced by
human selection is afforded by the various races and breeds of domestic
pigeons, not only because the variations produced are often most
extraordinary in amount and diverse in character, but because in this
case there is no doubt whatever that all have been derived from one wild
species, the common rock-pigeon (Columba livia). As this is a very
important point it is well to state the evidence on which the belief is
founded. The wild rock-pigeon is of a slaty-blue colour, the tail has a
dark band across the end, the wings have two black bands, and the outer
tail-feathers are edged with white at the base. No other wild pigeon in
the world has this combination of characters. Now in every one of the
domestic varieties, even the most extreme, all the above marks, even to
the white edging of the outer tail-feathers, are sometimes found
perfectly developed. When birds belonging to two distinct breeds are
crossed one or more times, neither of the parents being blue, or having
any of the above-named marks, the mongrel offspring are very apt to
acquire some of these characters. Mr. Darwin gives instances which he
observed himself. He crossed some white fantails with some black barbs,
and the mongrels were black, brown, or mottled. He also crossed a barb
with a spot, which is a white bird with a red tail and red spot on the
forehead, and the mongrel offspring were dusky and mottled. On now
crossing these two sets of mongrels with each other, he obtained a bird
of a beautiful blue colour, with the barred and white edged tail, and
double-banded wings, so as almost exactly to resemble a wild
rock-pigeon. This bird was descended in the second generation from a
pure white and pure black bird, both of which when unmixed breed their
kind remarkably true. These facts, well known to experienced
pigeon-fanciers, together with the habits of the birds, which all like
to nest in holes, or dovecots, not in trees like the great majority of
wild pigeons, have led to the general belief in the single origin of all
the different kinds.
In order to afford some idea of the great differences which exist among
domesticated pigeons, it will be well to give a brief abstract of Mr.
Darwin's account of them. He divides them into eleven distinct races,
most of which have several sub-races.
RACE I. _Pouters_.--These are especially distinguished by the enormously
enlarged crop, which can be so inflated in some birds as almost to
conceal the beak. They are very long in the body and legs and stand
almost upright, so as to present a very distinct appearance. Their
skeleton has become modified, the ribs being broader and the vertebrae
more numerous than in other pigeons.
RACE II. _Carriers_.--These are large, long-necked birds, with a long
pointed beak, and the eyes surrounded with a naked carunculated skin or
wattle, which is also largely developed at the base of the beak. The
opening of the mouth is unusually wide. There are several sub-races, one
being called Dragons.
RACE III. _Runts_.--These are very large-bodied, long-beaked pigeons,
with naked skin round the eyes. The wings are usually very long, the
legs long, and the feet large, and the skin of the neck is often red.
There are several sub-races, and these differ very much, forming a
series of links between the wild rock-pigeon and the carrier.
RACE IV. _Barbs_.--These are remarkable for their very short and thick
beak, so unlike that of most pigeons that fanciers compare it with that
of a bullfinch. They have also a naked carunculated skin round the eyes,
and the skin over the nostrils swollen.
RACE V. _Fantails_.--Short-bodied and rather small-beaked pigeons, with
an enormously developed tail, consisting usually of from fourteen to
forty feathers instead of twelve, the regular number in all other
pigeons, wild and tame. The tail spreads out like a fan and is usually
carried erect, and the bird bends back its slender neck, so that in
highly-bred varieties the head touches the tail. The feet are small, and
they walk stiffly.
RACE VI. _Turbits and Owls_.--These are characterised by the feathers of
the middle of neck and breast in front spreading out irregularly so as
to form a frill. The Turbits also have a crest on the head, and both
have the beak exceedingly short.
RACE VII. _Tumblers_.--- These have a small body and short beak, but
they are specially distinguished by the singular habit of tumbling over
backwards during flight. One of the sub-races, the Indian Lotan or
Ground tumbler, if slightly shaken and placed on the ground, will
immediately begin tumbling head over heels until taken up and soothed.
If not taken up, some of them will go on tumbling till they die. Some
English tumblers are almost equally persistent. A writer, quoted by Mr.
Darwin, says that these birds generally begin to tumble almost as soon
as they can fly; "at three months old they tumble well, but still fly
strong; at five or six months they tumble excessively; and in the second
year they mostly give up flying, on account of their tumbling so much
and so close to the ground. Some fly round with the flock, throwing a
clean summersault every few yards till they are obliged to settle from
giddiness and exhaustion. These are called Air-tumblers, and they
commonly throw from twenty to thirty summersaults in a minute, each
clear and clean. I have one red cock that I have on two or three
occasions timed by my watch, and counted forty summersaults in the
minute. At first they throw a single summersault, then it is double,
till it becomes a continuous roll, which puts an end to flying, for if
they fly a few yards over they go, and roll till they reach the ground.
Thus I had one kill herself, and another broke his leg. Many of them
turn over only a few inches from the ground, and will tumble two or
three times in flying across their loft. These are called House-tumblers
from tumbling in the house. The act of tumbling seems to be one over
which they have no control, an involuntary movement which they seem to
try to prevent. I have seen a bird sometimes in his struggles fly a yard
or two straight upwards, the impulse forcing him backwards while he
struggles to go forwards."[34]
The Short-faced tumblers are an improved sub-race which have almost lost
the power of tumbling, but are valued for possessing some other
characteristics in an extreme degree. They are very small, have almost
globular heads, and a very minute beak, so that fanciers say the head of
a perfect bird should resemble a cherry with a barleycorn stuck in it.
Some of these weigh less than seven ounces, whereas the wild rock-pigeon
weighs about fourteen ounces. The feet, too, are very short and small,
and the middle toe has twelve or thirteen instead of fourteen or fifteen
scutellae. They have often only nine primary wing-feathers instead of
ten as in all other pigeons.
RACE VIII. _Indian Frill-back_.--In these birds the beak is very short,
and the feathers of the whole body are reversed or turn backwards.
RACE IX. _Jacobin_.--These curious birds have a hood of feathers almost
enclosing the head and meeting in front of the neck. The wings and tail
are unusually long.
RACE X. _Trumpeter_.--Distinguished by a tuft of feathers curling
forwards over the beak, and the feet very much feathered. They obtain
their name from the peculiar voice unlike that of any other pigeon. The
coo is rapidly repeated, and is continued for several minutes. The feet
are covered with feathers so large as often to appear like little wings.
RACE XI. comprises _Laughers_, _Frill-backs_, _Nuns_, _Spots_, _and
Swallows_.--They are all very like the common rock-pigeon, but have each
some slight peculiarity. The Laughers have a peculiar voice, supposed to
resemble a laugh. The Nuns are white, with the head, tail, and primary
wing-feathers black or red. The Spots are white, with the tail and a
spot on the forehead red. The Swallows are slender, white in colour,
with the head and wings of some darker colour.
Besides these races and sub-races a number of other kinds have been
described, and about one hundred and fifty varieties can be
distinguished. It is interesting to note that almost every part of the
bird, whose variations can be noted and selected, has led to variations
of a considerable extent, and many of these have necessitated changes in
the plumage and in the skeleton quite as great as any that occur in the
numerous distinct species of large genera. The form of the skull and
beak varies enormously, so that the skulls of the Short-faced tumbler
and some of the Carriers differ more than any wild pigeons, even those
classed in distinct genera. The breadth and number of the ribs vary, as
well as the processes on them; the number of the vertebrae and the
length of the sternum also vary; and the perforations in the sternum
vary in size and shape. The oil gland varies in development, and is
sometimes absent. The number of the wing-feathers varies, and those of
the tail to an enormous extent. The proportions of the leg and feet and
the number of the scutellae also vary. The eggs also vary somewhat in
size and shape; and the amount of downy clothing on the young bird, when
first hatched, differs very considerably. Finally, the attitude of the
body, the manner of walking, the mode of flight, and the voice, all
exhibit modifications of the most remarkable kind.[35]
_Acclimatisation_.
A very important kind of variation is that constitutional change termed
acclimatisation, which enables any organism to become gradually adapted
to a different climate from the parent stock. As closely allied species
often inhabit different countries possessing very different climates, we
should expect to find cases illustrating this change among our
domesticated animals and cultivated plants. A few examples will
therefore be adduced showing that such constitutional variation does
occur.
Among animals the cases are not numerous, because no systematic attempt
has been made to select varieties for this special quality. It has,
however, been observed that, though no European dogs thrive well in
India, the Newfoundland dog, originating from a severe climate, can
hardly be kept alive. A better case, perhaps, is furnished by merino
sheep, which, when imported directly from England, do not thrive, while
those which have been bred in the intermediate climate of the Cape of
Good Hope do much better. When geese were first introduced into Bogota,
they laid few eggs at long intervals, and few of the young survived. By
degrees, however, the fecundity improved, and in about twenty years
became equal to what it is in Europe. According to Garcilaso, when fowls
were first introduced into Peru they were not fertile, whereas now they
are as much so as in Europe.
Plants furnish much more important evidence. Our nurserymen distinguish
in their catalogues varieties of fruit-trees which are more or less
hardy, and this is especially the case in America, where certain
varieties only will stand the severe climate of Canada. There is one
variety of pear, the Forelle, which both in England and France withstood
frosts that killed the flowers and buds of all other kinds of pears.
Wheat, which is grown over so large a portion of the world, has become
adapted to special climates. Wheat imported from India and sown in good
wheat soil in England produced the most meagre ears; while wheat taken
from France to the West Indian Islands produced either wholly barren
spikes or spikes furnished with two or three miserable seeds, while West
Indian seed by its side yielded an enormous harvest. The orange was very
tender when first introduced into Italy, and continued so as long as it
was propagated by grafts, but when trees were raised from seed many of
these were found to be hardier, and the orange is now perfectly
acclimatised in Italy. Sweet-peas (Lathyrus odoratus) imported from
England to the Calcutta Botanic Gardens produced few blossoms and no
seed; those from France flowered a little better, but still produced no
seed, but plants raised from seed brought from Darjeeling in the
Himalayas, but originally derived from England, flower and seed
profusely in Calcutta.[36]
An observation by Mr. Darwin himself is perhaps even more instructive.
He says: "On 24th May 1864 there was a severe frost in Kent, and two
rows of scarlet runners (Phaseolus multiflorus) in my garden, containing
390 plants of the same age and equally exposed, were all blackened and
killed except about a dozen plants. In an adjoining row of Fulmer's
dwarf bean (Phaseolus vulgaris) one single plant escaped. A still more
severe frost occurred four days afterwards, and of the dozen plants
which had previously escaped only three survived; these were not taller
or more vigorous than the other young plants, but they escaped
completely, with not even the tips of their leaves browned. It was
impossible to behold these three plants, with their blackened, withered,
and dead brethren all around them, and not see at a glance that they
differed widely in their constitutional power of resisting frost."
The preceding sketch of the variation that occurs among domestic animals
and cultivated plants shows how wide it is in range and how great in
amount; and we have good reason to believe that similar variation
extends to all organised beings. In the class of fishes, for example, we
have one kind which has been long domesticated in the East, the gold
and silver carps; and these present great variation, not only of colour
but in the form and structure of the fins and other external organs. In
like manner, the only domesticated insects, hive bees and silkworm
moths, present numbers of remarkable varieties which have been produced
by the selection of chance variations just as in the case of plants and
the higher animals.
_Circumstances favourable to Selection by Man._
It may be supposed, that the systematic selection which has been
employed for the purpose of improving the races of animals or plants
useful to man is of comparatively recent origin, though some of the
different races are known to have been in existence in very early times.
But Mr. Darwin has pointed out, that unconscious selection must have
begun to produce an effect as soon as plants were cultivated or animals
domesticated by man. It would have been very soon observed that animals
and plants produced their like, that seed of early wheat produced early
wheat, that the offspring of very swift dogs were also swift, and as
every one would try to have a good rather than a bad sort this would
necessarily lead to the slow but steady improvement of all useful plants
and animals subject to man's care. Soon there would arise distinct
breeds, owing to the varying uses to which the animals and plants were
put. Dogs would be wanted chiefly to hunt one kind of game in one part
of the country and another kind elsewhere; for one purpose scent would
be more important, for another swiftness, for another strength and
courage, for yet another watchfulness and intelligence, and this would
soon lead to the formation of very distinct races. In the case of
vegetables and fruits, different varieties would be found to succeed
best in certain soils and climates; some might be preferred on account
of the quantity of food they produced, others for their sweetness and
tenderness, while others might be more useful on account of their
ripening at a particular season, and thus again distinct varieties would
be established. An instance of unconscious selection leading to distinct
results in modern times is afforded by two flocks of Leicester sheep
which both originated from the same stock, and were then bred pure for
upwards of fifty years by two gentlemen, Mr. Buckley and Mr. Burgess.
Mr. Youatt, one of the greatest authorities on breeding domestic
animals, says: "There is not a suspicion existing in the mind of any one
at all acquainted with the subject that the owner of either of them has
deviated in any one instance from the pure blood of Mr. Bakewell's
original flock, and yet the difference between the sheep possessed by
these two gentlemen is so great that they have the appearance of being
quite different varieties." In this case there was no desire to deviate
from the original breed, and the difference must have arisen from some
slight difference of taste or judgment in selecting, each year, the
parents for the next year's stock, combined perhaps with some direct
effect of the slight differences of climate and soil on the two farms.
Most of our domesticated animals and cultivated plants have come to us
from the earliest seats of civilisation in Western Asia or Egypt, and
have therefore been the subjects of human care and selection for some
thousands of years, the result being that, in many cases, we do not know
the wild stock from which they originally sprang. The horse, the camel,
and the common bull and cow are nowhere found in a wild state, and they
have all been domesticated from remote antiquity. The original of the
domestic fowl is still wild in India and the Malay Islands, and it was
domesticated in India and China before 1400 B.C. It was introduced into
Europe about 600 B.C. Several distinct breeds were known to the Romans
about the commencement of the Christian era, and they have since spread
all over the civilised world and been subjected to a vast amount of
conscious and unconscious selection, to many varieties of climate and to
differences of food; the result being seen in the wonderful diversity of
breeds which differ quite as remarkably as do the different races of
pigeons already described.
In the vegetable kingdom, most of the cereals--wheat, barley, etc.--are
unknown as truly wild plants; and the same is the case with many
vegetables, for De Candolle states that out of 157 useful cultivated
plants thirty-two are quite unknown in a wild state, and that forty more
are of doubtful origin. It is not improbable that most of these do exist
wild, but they have been so profoundly changed by thousands of years of
cultivation as to be quite unrecognisable. The peach is unknown in a
wild state, unless it is derived from the common almond, on which point
there is much difference of opinion among botanists and horticulturists.
The immense antiquity of most of our cultivated plants sufficiently
explains the apparent absence of such useful productions in Australia
and the Cape of Good Hope, notwithstanding that they both possess an
exceedingly rich and varied flora. These countries having been, until a
comparatively recent period, inhabited only by uncivilised men, neither
cultivation nor selection has been carried on for a sufficiently long
time. In North America, however, where there was evidently a very
ancient if low form of civilisation, as indicated by the remarkable
mounds, earthworks, and other prehistoric remains, maize was cultivated,
though it was probably derived from Peru; and the ancient civilisation
of that country and of Mexico has given rise to no fewer than
thirty-three useful cultivated plants.
_Conditions favourable to the production of Variations._
In order that plants and animals may be improved and modified to any
considerable extent, it is of course essential that suitable variations
should occur with tolerable frequency. There seem to be three conditions
which are especially favourable to the production of variations: (1)
That the particular species or variety should be kept in very large
numbers; (2) that it should be spread over a wide area and thus
subjected to a considerable diversity of physical conditions; and (3)
that it should be occasionally crossed with some distinct but closely
allied race. The first of these conditions is perhaps the most
important, the chance of variations of any particular kind being
increased in proportion to the quantity of the original stock and of its
annual offspring. It has been remarked that only those breeders who keep
large flocks can effect much improvement; and it is for the same reason
that pigeons and fowls, which can be so easily and rapidly increased,
and which have been kept in such large numbers by so great a number of
persons, have produced such strange and numerous varieties. In like
manner, nurserymen who grow fruit and flowers in large quantities have a
great advantage over private amateurs in the production of new
varieties.
Although I believe, for reasons which will be given further on, that
some amount of variability is a constant and necessary property of all
organisms, yet there appears to be good evidence to show that changed
conditions of life tend to increase it, both by a direct action on the
organisation and by indirectly affecting the reproductive system. Hence
the extension of civilisation, by favouring domestication under altered
conditions, facilitates the process of modification. Yet this change
does not seem to be an essential condition, for nowhere has the
production of extreme varieties of plants and flowers been carried
farther than in Japan, where careful selection continued for many
generations must have been the chief factor. The effect of occasional
crosses often results in a great amount of variation, but it also leads
to instability of character, and is therefore very little employed in
the production of fixed and well-marked races. For this purpose, in
fact, it has to be carefully avoided, as it is only by isolation and
pure breeding that any specially desired qualities can be increased by
selection. It is for this reason that among savage peoples, whose
animals run half wild, little improvement takes place; and the
difficulty of isolation also explains why distinct and pure breeds of
cats are so rarely met with. The wide distribution of useful animals and
plants from a very remote epoch has, no doubt, been a powerful cause of
modification, because the particular breed first introduced into each
country has often been kept pure for many years, and has also been
subjected to slight differences of conditions. It will also usually have
been selected for a somewhat different purpose in each locality, and
thus very distinct races would soon originate.
The important physiological effects of crossing breeds or strains, and
the part this plays in the economy of nature, will be explained in a
future chapter.
_Concluding Remarks._
The examples of variation now adduced--and these might have been almost
indefinitely increased--will suffice to show that there is hardly an
organ or a quality in plants or animals which has not been observed to
vary; and further, that whenever any of these variations have been
useful to man he has been able to increase them to a marvellous extent
by the simple process of always preserving the best varieties to breed
from. Along with these larger variations others of smaller amount
occasionally appear, sometimes in external, sometimes in internal
characters, the very bones of the skeleton often changing slightly in
form, size, or number; but as these secondary characters have been of no
use to man, and have not been specially selected by him, they have,
usually, not been developed to any great amount except when they have
been closely dependent on those external characters which he has largely
modified.
As man has considered only utility to himself, or the satisfaction of
his love of beauty, of novelty, or merely of something strange or
amusing, the variations he has thus produced have something of the
character of monstrosities. Not only are they often of no use to the
animals or plants themselves, but they are not unfrequently injurious to
them. In the Tumbler pigeons, for instance, the habit of tumbling is
sometimes so excessive as to injure or kill the bird; and many of our
highly-bred animals have such delicate constitutions that they are very
liable to disease, while their extreme peculiarities of form or
structure would often render them quite unfit to live in a wild state.
In plants, many of our double flowers, and some fruits, have lost the
power of producing seed, and the race can thus be continued only by
means of cuttings or grafts. This peculiar character of domestic
productions distinguishes them broadly from wild species and varieties,
which, as will be seen by and by, are necessarily adapted in every part
of their organisation to the conditions under which they have to live.
Their importance for our present inquiry depends on their demonstrating
the occurrence of incessant slight variations in all parts of an
organism, with the transmission to the offspring of the special
characteristics of the parents; and also, that all such slight
variations are capable of being accumulated by selection till they
present very large and important divergencies from the ancestral stock.
We thus see, that the evidence as to variation afforded by animals and
plants under domestication strikingly accords with that which we have
proved to exist in a state of nature. And it is not at all surprising
that it should be so, since all the species were in a state of nature
when first domesticated or cultivated by man, and whatever variations
occur must be due to purely natural causes. Moreover, on comparing the
variations which occur in any one generation of domesticated animals
with those which we know to occur in wild animals, we find no evidence
of greater individual variation in the former than in the latter. The
results of man's selection are more striking to us because we have
always considered the varieties of each domestic animal to be
essentially identical, while those which we observe in a wild state are
held to be essentially diverse. The greyhound and the spaniel seem
wonderful, as varieties of one animal produced by man's selection; while
we think little of the diversities of the fox and the wolf, or the horse
and the zebra, because we have been accustomed to look upon them as
radically distinct animals, not as the results of nature's selection of
the varieties of a common ancestor.
FOOTNOTES:
[Footnote 31: Darwin, _Animals and Plants under Domestication_, vol. i.
p. 322.]
[Footnote 32: These facts are taken from Darwin's _Domesticated Animals
and Cultivated Plants_, vol. i. pp. 359, 360, 392-401; vol. ii. pp. 231,
275, 330.]
[Footnote 33: See Darwin's _Animals and Plants under Domestication_,
vol. i. pp. 40-42.]
[Footnote 34: Mr. Brent in _Journal of Horticulture_, 1861, p. 76;
quoted by Darwin, _Animals and Plants under Domestication_, vol. i. p.
151.]
[Footnote 35: This account of domestic pigeons is greatly condensed from
Mr. Darwin's work already referred to.]
[Footnote 36: _Animals and Plants under Domestication_, vol. ii. pp.
307-311.]