ES AND THE USUAL
STERILITY OF THEIR HYBRID OFFSPRING

Statement of the problem--Extreme susceptibility of the
reproductive functions--Reciprocal crosses--Individual
differences in respect to cross-fertilisation--Dimorphism and
trimorphism among plants--Cases of the fertility of hybrids and
of the infertility of mongrels--The effects of close
interbreeding--Mr. Huth's objections--Fertile hybrids among
animals--Fertility of hybrids among plants--Cases of sterility
of mongrels--Parallelism between crossing and change of
conditions--Remarks on the facts of hybridity--Sterility due to
changed conditions and usually correlated with other
characters--Correlation of colour with constitutional
peculiarities--The isolation of varieties by selective
association--The influence of natural selection upon sterility
and fertility--Physiological selection--Summary and concluding
remarks.

One of the greatest, or perhaps we may say the greatest, of all the
difficulties in the way of accepting the theory of natural selection as
a complete explanation of the origin of species, has been the remarkable
difference between varieties and species in respect of fertility when
crossed. Generally speaking, it may be said that the varieties of any
one species, however different they may be in external appearance, are
perfectly fertile when crossed, and their mongrel offspring are equally
fertile when bred among themselves; while distinct species, on the other
hand, however closely they may resemble each other externally, are
usually infertile when crossed, and their hybrid offspring absolutely
sterile. This used to be considered a fixed law of nature, constituting
the absolute test and criterion of a _species_ as distinct from a
_variety_; and so long as it was believed that species were separate
creations, or at all events had an origin quite distinct from that of
varieties, this law could have no exceptions, because, if any two
species had been found to be fertile when crossed and their hybrid
offspring to be also fertile, this fact would have been held to prove
them to be not _species_ but _varieties_. On the other hand, if two
varieties had been found to be infertile, or their mongrel offspring to
be sterile, then it would have been said: These are not varieties but
true species. Thus the old theory led to inevitable reasoning in a
circle; and what might be only a rather common