84 CONDITIONS AFFECTING THE 



" hybrids," which are crosses between distinct species. 

 The mongrels are, so far as we know, fertile with one 

 another. But between species, in many cases, you cannot 

 succeed in obtaining even the first cross : 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 Fantail 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 well to talk about 

 producing these different races, but you know very well 

 that if you turned all these birds wild, these Pouters, and 

 Carriers, and so on, they would all return to their primitive 

 stock." This is very commonly assumed to be a fact, and 

 it is an argument that is commonly brought forward as 

 conclusive ; but if you will take the trouble to inquire 

 into it rather closely, I think you will find that it is not 

 worth very much. The first question of course is, Do they 

 thus return to the primitive stock ? And commonly as 



