142 THE GENESIS OF SPECIES. [CHAP. 



common parent, should produce an effect equal to that of 

 captivity, and consequent change of habit, as well as con- 

 siderable modification of food. 



No clear case has been given by Mr. Darwin in which 

 mongrel animals, descended from the same undoubted 

 species, have been persistently infertile inter se ; nor any 

 clear case in which hybrids between animals generally 

 admitted to be of distinct species, have been continuously 

 fertile inter se. 



It is true that facts are brought forward tending to 

 establish the probability of the doctrine of Pallas, that 

 species may sometimes be rendered fertile by domestica- 

 tion. But even if this were true, it would be no approxi- 

 mation towards proving the converse, i.e. that races and 

 varieties may become sterile when wild. Arid whatever 

 may be the preference occasionally shown by certain breeds 

 to mate with their own variety, no sterility is recorded as 

 resulting from unions with other varieties. Indeed, Mr. 

 Darwin remarks : l " With respect to sterility from the 

 crossing of domestic races, I know of no well-ascertained 

 case with animals. This fact (seeing the great difference 

 in structure between some breeds of pigeons, fowls, pigs, 

 dogs, &c.) is extraordinary when contrasted with the 

 sterility of many closely-allied natural species when 

 crossed." 



It has been alleged that the domestic and wild guinea- 

 pig do not breed together, but the specific identity of these 

 forms is very problematical. Mr. A. D. Bartlett, super- 

 intendent of the Zoological Gardens, whose experience is 

 so great, and observation so quick, believes them to be of 

 decidedly distinct species. 



1 "Animals and Plants under Domestication," vol. ii. p. 104. 



