He shows us that crosses between
some varieties are infertile or even sterile, while crosses between some
species are quite fertile; and that there are besides a number of
curious phenomena connected with the subject which render it impossible
to believe that sterility is anything more than an incidental property
of species, due to the extreme delicacy and susceptibility of the
reproductive powers, and dependent on physiological causes we have not
yet been able to trace. Nevertheless, the fact remains that most species
which have hitherto been crossed produce sterile hybrids, as in the
well-known case of the mule; while almost all domestic varieties, when
crossed, produce offspring which are perfectly fertile among themselves.
I will now endeavour to give such a sketch of the subject as may enable
the reader to see something of the complexity of the problem, referring
him to Mr. Darwin's works for fuller details.
_Extreme Susceptibility of the Reproductive Functions._
One of the most interesting facts, as showing how susceptible to changed
conditions or to slight constitutional changes are the reproductive
powers of animals, is the very general difficulty of getting those which
are kept in confinement to breed; and this is frequently the only bar to
domesticating wild species.
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