PERPETUATION OF LIVING BEINGS. 113 



at any rate it is quite certain that the hybrids are often 

 absolutely infertile one with another. 



Here is a feature^ then, great or small as it may be, 

 which distinguishes natural species of animals. Can 

 we find any approximation to this in the different races 

 known to be produced by selective breeding from a 

 common stock ? Up to the present time the answer to 

 that question is absolutely a negative one. As far as we 

 know at present, there is nothing approximating to 

 this check. In crossing the breeds between the Fan- 

 tail and the Pouter, the Carrier and the Tumbler, or 

 any other variety or race you may name — so far as we 

 know at present — there is no difficulty in breeding 

 together the mongrels. Take the Carrier and the 

 Fantail, for instance, and let them represent the Horse 

 and the Ass in the case of distinct species ; then you have, 

 as the result of their breeding, the Carrier-Fantail 

 mongrel, — we will say the male and female mongrel, — 

 and, as far as we know, these two when crossed would 

 not be less fertile than the original cross, or than Carrier 

 with Carrier. Here, you see, is a physiological contrast 

 between the races produced by selective modification and 

 natural species. I shall inquire into the value of this 

 fact, and of some modifying circumstances by and by ; 

 for the present I merely put it broadly before you. 



But while considering this question of the limitations 

 of species, a word must be said about what is called 

 Recurrence — the tendency of races which have been 

 developed by selective breeding from varieties to 

 return to their primitive type. This is supposed by 

 many to put an absolute limit to the extent of selective 

 and all other variations. People say, " It is all very 



