CHAPTEE YII 



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 iuter-breeding — Mr. Huth's objections — Fertile 

 liybrids 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. 



V One of the greatest, or perhaps Ave may say the greatest, of 



^^ all the difficulties in the Avay of accepting the theory of 

 ^ natural selection as a complete exj^lanation of the origin of 



species, has been the remarkable difllerence between varieties 

 and species in respect of fertility when crossed. Generally 

 sjjeaking, 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 

 ecjually fertile Avhen bred among themselves ; Avhile 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 Avere separate creations, or 



