CHAPTER VII 
ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES 
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 inter-breeding—Mr. Hutli’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 ore 
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 lie 
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 
