CHAP. VII ON THE INFERTILITY OF CROSSES 153 



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 

 ofispring 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 mongi-el 

 offspring to be sterile, then it would have been said : These 

 are not A^arieties 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 Avhich had no j 

 exceptions. 



The elaborate and careful examination of the Avhole subject 

 by Mr. Darwin, who has brought together a vast mass of 

 evidence from the experience of agriculturists and horti- J 

 culturists, as Avell as from scientific experimenters, has demon- 1 

 strated that there is no such fixed law in nature as was I 

 formerly supposed. He shows us that crosses between some 

 varieties are infertile or oven sterile, while crosses betAveen 

 some species are quite fertile ; and that there are besides a 

 number of curious i^henomena connected with the subject 

 Avhich render it impossible to believe that sterility is anything 

 more than an incidental property of species, due to the 

 extreme delicacy and susceptibility of the reproductiA^e powers, 

 and dependent on physiological causes Ave have not yet been 

 able to trace. Nevertheless, the fact remains that most species 

 Avhich have hitherto been crossed produce sterile hybrids, as 

 in the Avell-knoAvn case of the mule ; Avhile almost all domestic 

 varieties, Avhen crossed, produce offspring Avhich are perfectly 

 fertile among themselves. I Avill noAV endeavour to give such 

 a sketch of the subject as may enable the reader to see some- 

 thing of the complexity of the problem, referring him to ]\Tr. 

 DarAvin's Avorks for fuller details. 



Extreme Susceptibility of the Reproductive Functions. 



One of the most interesting facts, as shoAving hoAV sus- 

 ceptible to changed conditions or to slight constitutional 

 changes are the reproductiA'e poAA^ers of animals, is the very 

 general difficulty of getting those AAdiich are kept in confine- 

 ment to breed ; and this is frequently the only bar to 

 domesticating Avild species. Thus, elephants, bears, foxes, 



