CHAP. VII OX THE IX FERTILITY 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 

 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 ine^^table reasoning in a circle ; and what might be only a 

 rather common fact was elevated into a law which had no 

 exception:^. 



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 agTiculturists and horti- 

 culturists, as well as from scientific experimenters, has demon- 

 strated that there is no such fixed laAv in natiu'e as was 

 formerly supposed. He shows us that crosses between some 

 varieties are infertile or even sterile, while crosses between 

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

 number of cuiious phenomena 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 reproductive powers, 

 and dependent on physiological causes we have not yet been 

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

 which have hitherto been crossed produce sterile hybrids, as 

 in the well-known case of the mule ; while almost all domestic 

 varieties, when crossed, produce offspring which are perfectly 

 fertile among themselves. I will now 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 ]\Ir. 

 Darv.in's Avorks for fuller details. 



Extreme SuscepHhilitij of the Eeproductive Functions. 

 One of the most interestins; facts, as showing; how sus- 

 ceptible to changed conditions or to slight constitutional 

 changes are the reproductive powers of animals, is the very 

 general difficulty of getting those which are kept in confine- 

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

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



