the period of maturity, including that of the
second dentition, has been much hastened. The period of gestation varies
much, but has been modified in a fixed manner in only one or two cases. In
{410} our poultry and pigeons the acquirement of down and of the first
plumage by the young, and of the secondary sexual characters by the males,
differ. The number of moults through which the larvae of silk-moths pass,
varies. The tendency to fatten, to yield much milk, to produce many young
or eggs at a birth or during life, differs in different breeds. We find
different degrees of adaptation to climate, and different tendencies to
certain diseases, to the attacks of parasites, and to the action of certain
vegetable poisons. With plants, adaptation to certain soils, as with some
kinds of plums, the power of resisting frost, the period of flowering and
fruiting, the duration of life, the period of shedding the leaves and of
retaining them throughout the winter, the proportion and nature of certain
chemical compounds in the tissues or seeds, all vary.

There is, however, one important constitutional difference between domestic
races and species; I refer to the sterility which almost invariably
follows, in a greater or less degree, when species are crossed, and to the
perfect fertility of the most distinct domestic races, with the exception
of a very few plants, when similarly crossed. It certainly appears a
remarkable fact that many closely allied species which in appearance differ
extremely little should yield when united only a few, more or less sterile
offspring, or none at all; whilst domestic races which differ conspicuously
from each other, are when united remarkably fertile, and yield perfectly
fertile offspring. But this fact is not in reality so inexplicable as it at
first appears. In the first place, it was clearly shown in the nineteenth
chapter that the sterility of crossed species does not closely depend on
differences in their external structure or general constitution, but
results exclusively from differences in the reproductive system, analogous
with those which cause the lessened fertility of the illegitimate unions
and illegitimate offspring of dimorphic and trimorphic plants. In the
second place, the Pallasian doctrine, that species after having been long
domesticated lose their natural tendency to sterility when crossed, has
been shown to be highly probable; we can scarcely avoid this conclusion
when we reflect