A Biological Bogey 
cidental on unknown peculiarities of the repro- 
ductive system. These peculiarities constantly 
tend to arise under changed conditions owing 
to the extreme susceptibility of that system, and 
they are usually correlated with variations of 
form or of colour. Hence, as fixed differences 
of form and colour, slowly gained by natural 
selection in adaptation to changed conditions, 
are what essentially characterise distinct species, 
some amount of infertility between species is 
the usual result.” 
But Wallace has not been content to let the 
matter remain where Darwin left it. He has 
boldly tried to make an ally of this bogey of the 
infertility of hybrids. On page 179 of Darwinism 
he argues, most ingeniously, that the sterility of 
hybrids has been actually produced by natural 
selection to prevent the evils of the intercrossing 
of allied species. We will not reproduce his 
argument for the simple reason that it is now 
well-known, or should be well-known, that hybrids 
between allied species are by no means always 
sterile. The doctrine of the infertility of hybrids 
seems to have been founded on the fact that the 
hybrids best known to breeders, namely the 
cross between the ass and the horse, and those 
between the canary and other finches, are sterile. 
117 
