140 THE GENESIS OF SPECIES. [CHAP. 



tured, are often rendered sterile by captivity, so the influ-: 

 ence of union with another species may produce a similar 

 effect. It seems to the author an unwarrantable assump- 

 tion that a cross with what, on the Darwinian theory, can 

 only be a slightly-diverging descendant of a common par- 

 ent, should produce an effect equal to that of captivity, 

 and consequent change of habit, as well as considerable 

 modification of food. 



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

 mongrel animals, descended from the same undoubted spe- 

 cies, have been persistently infertile inter se / nor any clear 

 case in which hybrids between animals, generally admitted 

 to be distinct species, have been continually fertile inter se. 



It is true that facts are brought forward tending to 

 establish the probability of the doctrine of Pallas, that spe- 

 cies may sometimes be rendered fertile by domestication. 

 But even if this were true, it would be no approximation 

 toward proving the converse, i. e., that races and varieties 

 may become sterile when wild. And 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 re- 

 marks, 21 "With respect to sterility from the crossing of 

 domestic races, I know of no well-ascertained case with ani- 

 mals. This fact (seeing the great -difference in structure 

 between some breeds of pigeons, fowls, pigs, dogs, etc.) 

 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, superin- 

 tendent of the Zoological Gardens, whose experience is 

 so great, and observation so quick, believes them to be de- 

 cidedly distinct species. 



21 " Animals and Plants under Domestication," voL ii., p. 104. 



