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.
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