104 Animal Life and Intelligence. 



a chance sport in a single lamb. The offspring of this 

 ram were either ancons or ordinary sheep. The ancon 

 characters did not blend. Hence for a time a definite 

 breed was maintained. We may call this mode of isola- 

 tion isolation by exclusive inheritance. 



A further mode of isolation or segregation, for which 

 Mr. Eomanes * claims a foremost, indeed, the foremost, 

 place, is physiological isolation as due to differential fertility. 

 One among the many variations to which organisms are 

 subject is a variation in fertility, which may reach the 

 climax of absolute sterility. But it is clear that a sterile 

 variation carries with it its own death-warrant, since the 

 sterile individual leaves no descendants to inherit its pecu- 

 liarity. Eelative infertility, too, unless it chances to be 

 correlated with some unusual excellence, would be no 

 advantage, would be transmitted to few descendants, and 

 would tend to be extinguished. The same is not true, 

 however, of differential fertility. "It is by no means 

 rare," said Darwin, f "to find certain males and females 

 which will not breed together, though both are known to 

 be perfectly fertile with other males and females." Mr. 

 Eomanes assumes, as a starting-point, the converse of 

 this, namely, that certain males and females will breed to- 

 gether, though they are infertile with all other members 

 of the species. 



Suppose, then, a variety to arise which is perfectly 

 fertile within the limits of the varietal form, but im- 

 perfectly fertile or infertile with the parent species. 

 Such a variety would have to run the risks of those ill 

 effects which, as Darwin showed, t are attendant upon close 

 interbreeding. But Mr. Wallace points out§ that these 

 ill effects may not be so marked under nature as they are 

 under domestication. Suppose, then, that it escapes these 

 ill effects. In this case, Mr. Eomanes urges, it would 

 neither be swamped by intercrossing nor die out on 



* Journal of the Linnxan Society, vol. xix. No. 115 : " Zoology." 

 t " Animals and Plants under Domestication," p. 145. 

 % Ibid. chap. xvii. § " Darwinism," p. 326. 



