The Descent of Man

laws, for instance, that of the unequal fertility
of converse crosses between the same two species.
With forms which must be ranked as undoubted
species, a perfect series exists from
those which are absolutely sterile when crossed,
to those which are almost or completely
fertile. The degrees of sterility do not coincide
strictly with the degrees of difference between
the parents in external structure or habits of
life. Man in many respects may be compared
with those animals which have long been domesticated,
and a large body of evidence can be
advanced in favour of the Pallasian doctrine
(14. The 'Variation of Animals and Plants under
Domestication,' vol. ii. p. 109. I may here remind
the reader that the sterility of species
when crossed is not a specially-acquired quality,
but, like the incapacity of certain trees to be
grafted together, is incidental on other acquired
differences. The nature of these differences is
unknown, but they relate more especially to the
reproductive system, and much less so to ex-