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 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 fact was elevated into a law which had no exceptions.
The elaborate and careful examination of the whole subject by Mr.
Darwin, who has brought together a vast mass of evidence from the experience of agriculturists and horticulturists, as well as from scientific experimenters, has demonstrated that there is no such fixed law in nature as was formerly supposed.