ME. G. J. EOMANES ON PHYSIOLOGICAL SELECTION. 357 



but must be perpetuated by the ever vigilant agency of pbysiolo- 

 gical selection. What this particular variation is we now know, 

 and I will proceed to give evidence of its spontaneous occurrence, 

 first in individuals, second in varieties, and tbird in species. 



1. Individimls. — Mr. Darwin observes : — "It is by no means rare 

 to find certain males and females wbich will not breed together, 

 though both are known to be perfectly fertile with other males 

 and females. We have no reason to suppose that this is caused 

 by these animals having been subject to any change in their habits 

 of life ; therefore such cases are hardly related to our present 

 subject. The cause apparently lies in an innate sexual incom- 

 patibility of the pair when matched." He then proceeds to give 

 examples from horses, cattle, pigs, dogs, and pigeons, concluding 

 with the remark that " these facts are worth recording, as they 

 show, like so many previous facts, on what slight constitutional 

 difierences the fertility of an animal often depends " *. And in 

 another place he gives references to similar facts in the case of 

 plants t- 



Now, if it were needful, I could supply a number of additional 

 eases of this individual incompatibility, or of absolute sterility as 

 between two individuals, each of which is perfectly fertile with all 

 other individuals %. But I think that the not unusual occurrence 

 of this fact will be regarded as abundantly substantiated by these 

 references. 



And here, it appears to me, we have a most significant piece of 

 evidence upon the origin of species. If even as between two 

 individuals there may thus arise absolute sterility, without there 

 being in either of them the least impairment of fertility with 

 other individuals, is it not obvious that we have precisely the kind 

 of variation which my theory requires, and that we have this vari- 

 ation spontaneously or suddenly given in the highest possible 

 degree of efficiency ? Shallow criticism might reply that this is 

 the precise opposite of the variation which my theory requires ; 

 and under one point of view such is the case. For here we have 



* ' Variation,' &c., vol. ii. pp. 145-6. t ' Origin of Species,' ed. 6, p. 246. 



\ I may remark that individual incompatibility is especially apt to declare 

 itself when the individuals paired belong to difierent species. That is to say, 

 while some individuals taken from the two species will readily produce hybrids, 

 other individuals taken from the same species will prove hopelessly sterile. The 

 same applies to the fertility of hybrids. These facts are of some additional 

 importance to us, because they occupy a kind of intermediate position between 

 these given above and those given in the next succeeding paragraphs. 



