THE ORIGIN OF SPECIES 6th EDITION

CHAPTER I. VARIATION UNDER DOMESTICATION.

Causes of Variability--Effects of Habit and the use and disuse of
Parts--Correlated Variation--Inheritance--Character of Domestic
Varieties--Difficulty of distinguishing between Varieties and
Species--Origin of Domestic Varieties from one or more Species--Domestic
Pigeons, their Differences and Origin--Principles of Selection,
anciently followed, their Effects--Methodical and Unconscious
Selection--Unknown Origin of our Domestic Productions--Circumstances
favourable to Man's power of Selection.


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 in directly 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 incidently 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 in
definite. 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, etc. 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, shows us what singular modifications might result in the case of
plants from a chemical change in the nature of the sap.

In definite 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 in
definite 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.