STERILITY OF HYBRIDS 241 



interpret this fact as meaning not so much that the apple and 

 the pear are in reality very wide apart, but rather that either, 

 each is lacking in one of two complementary elements, or that 

 each possesses a factor with an inhibitory effect. Their incom- 

 patibility may well be of the same nature as that of the classes 

 in Cardamine pratensis. 



Returning now to the problem of inter-specific sterility; we 

 note, as I have said, the absence of contemporary evidence that 

 variation can confer on a variety the power to form a sterile 

 hybrid with the parent species. The considerations based on 

 this want of evidence have for a long while been familiar to all 

 who have discussed evolutionary theories, and it is worth observ- 

 ing the exact reason why the difficulty strikes us now with a new 

 and special force. In pre-Mendelian times all that was known 

 was that some forms could freely interbreed without diminution 

 of fertility in the product, while others could not. But now we 

 find that, by virtue of segregation, from one and the same pair 

 of parents, or even, in the case of hermaphrodites, from one and 

 the same individual, offspring commonly arises howing among 

 themselves exactly such differences as distinguish species and 

 very good species too. This we see happening again and again. 

 But to forms capable of arising as brethren in one family the 

 title species has never been meant to apply, and if we are going 

 to use the term in application to fraternal groups we must 

 definitely recognise that by " specific " difference is to be under- 

 stood simply difference, without any immediate or even ulterior 

 physiological limitation whatever. Naturally, therefore, we begin 

 to think of the appearance of sterility in crosses as something 

 apart, and as a manifestation which distinguishes certain kinds 

 of unions in a very special way. 



I am perfectly aware that there are gradations in the sterility 

 of hybrids as in every other characteristic upon which it has been 

 proposed to base specific definitions; but, as also so often happens 

 in the matter of defining intergrading categories, the difficulty 

 in practice is not often such as to lead to actual ambiguity. I 

 am speaking of course of those examples which are amenable to 

 genetic experiment. As to the rest there is complete and perma- 



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