424 THE CAUSES OF THE xi 



it is quite certain that the hybrids are often abso- 

 lutely 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 abso- 

 lutely 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 repre- 

 sent the Horse and the Ass in the case of distinct 

 species ; then you have, as the result of their breed- 

 ing, the Carrier-Fan tail 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 Car- 

 rier. Here, you see, is a physiological contrast 

 between the races produced by selective modifica- 

 tion and natural species. I shall inquire into the 

 value of this fact, and of some modifying circum- 

 stances by and by ; for the present I merely put 

 it broadly before you. 



But while considering this question of the limi- 

 tations of species, a word must be said about what 



