CHAPTER I
VARIATION UNDER DOMESTICATION
Causes of Variability
WHEN we compare the individuals of the same variety or sub-variety
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. There is, also, some probability in the
view propounded by Andrew Knight, that this variability may be
partly connected with excess of food. It seems clear that organic
beings must be exposed during several generations to new conditions to
cause any great amount of variation; and that, when the organisation
has once begun to vary, it generally continues varying for many
generations. No case is on record of a variable organism ceasing to
vary under cultivation. Our oldest cultivated plants, such as wheat,
still yield new varieties: our oldest, domesticated animals are
still capable of rapid improvement or modification.
As far as I am able to judge, after long attending to the subject,
the conditions of life appear to act in two ways,- directly on the
whole organisation or on certain parts alone, and indirectly by
affecting the reproductive system. With respect to the direct
action, we must bear in mind that in every case, as Professor Weismann
has lately insisted, and as I have incidentally shown in my work on
Variation under Domestication, there are two factors: namely, the
nature of the organism, and the nature of the conditions. The former
seems to be much the more important; for nearly similar variations
sometimes arise under, as far as we can judge, dissimilar
conditions; and, on the other hand, dissimilar variations arise
under conditions which appear to be nearly uniform. The effects on the
offspring are either definite or indefinite. They may be considered as
definite when all or nearly all the offspring of individuals exposed
to certain conditions during several generations are modified in the
same manner. It is extremely difficult to come to any conclusion in
regard to the extent of the changes which have been thus definitely
induced. There can, however, be little doubt about many slight
changes,- such as size from the amount of food, colour from the nature
of the food, thickness of the skin and hair from climate, &c. Each
of the endless variations which we see in the plumage of our fowls
must have had some efficient cause; and if the same cause were to
act uniformly during a long series of generations on. many
individuals, all probably would be modified in the same manner. Such
facts as the complex and extraordinary out-growths which variably
follow from the insertion of a minute drop of poison by a
gall-producing insect, show us what singular modifications might
result in the case of plants from a chemical change in the nature of
the sap.
Indefinite variability is a much more common result of changed
conditions than definite variability, and has probably played a more
important part in the formation of our domestic races. We see
indefinite variability in the endless slight peculiarities which
distinguish the individuals of the same species, 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 monstrosities cannot be
separated by any distinct line from slighter variations. All such
changes of structure, whether extremely slight or strongly marked,
which appear amongst many individuals living together, may be
considered as the indefinite 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, rheumatism, or
inflammation of various organs.
With respect to what I have called the indirect action of changed
conditions, namely, through the reproductive system of being affected,
we may infer that variability is thus induced, partly from the fact of
this system being extremely sensitive to any change in the conditions,
and partly from the similarity, as Kreuter 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 vitiated instincts. Many cultivated
plants display the utmost vigour, 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 subject; but to show how singular the laws
are which determine the reproduction of animals under confinement, I
may mention that carnivorous animals, even 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
condition as in the most sterile hybrids. When, on the one hand, we
see domesticated 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 affected
by unperceived causes as to fail to act, we need not be surprised at
this system, when it does act under confinement, acting irregularly,
and producing offspring somewhat unlike their parents. I may add, that
as some organisms breed freely under the most unnatural conditions
(for instance, rabbits and ferrets kept in hutches), showing that
their reproductive organs are not easily affected; so will some
animals and plants withstand domestication 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 gardeners;- 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, &c., and sometimes by seed. They occur rarely under
nature, but are far from rare under culture. As a single bud out of
the 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
combustible matter is ignited, has in determining the nature of the
flames.
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