298 THE VARIATION OF ANIMALS IN NATURE 



of permanent isolation would probably admit that the observed 

 differences in these organs are the result of more than one 

 evolutionary step, except, perhaps, in some of the least modified 

 geographical races. Further, there is no suggestion that any 

 environmental influence has played a direct part in the specific 

 modifications of the genitalia. This must be due to the 

 spread through the population of small variations, occurring 

 at first in a few individuals. The most obvious agency to 

 account for such a spread would be Natural Selection. Each 

 race of any widespread species might be so well adapted to its 

 own area that individuals capable of crossing (with the pro- 

 duction of intermediate forms unfitted to either one area or 

 the other) would be at a discount. A theory very much on 

 these lines has been propounded by Fisher (1930, pp. 125-31). 

 He suggests, first, that any species spread over a considerable 

 geographical area will tend to be differentiated at each end of 

 its range into a locally adapted form which will at first be 

 connected by a complete series of intermediates. In the course 

 of time the end-forms would get more and more unlike and 

 each more and more unfitted to live in the area inhabited by 

 the other. The process of diffusion from one end to the other 

 would gradually be retarded by the operation of selection, 

 since the individuals with the strongest tendency to migrate to 

 the parts of the range to which they were ill adapted would 

 be eliminated. Further, any preference shown by individuals 

 of one type for individuals like themselves will be advantageous, 

 since it will lead to an intensification of local adaptation and 

 will tend to stop locally adapted individuals from crossing with 

 less-adapted migrants into their area. There might thus be 

 built up a sexual preference which would hasten the process 

 of fission and eventually make it permanent. 



It should be noted that this explanation is purely formal 

 and no example is forthcoming, as might be expected from 

 the difficulty of obtaining the necessary evidence. In our 

 opinion such a process is unlikely to be very prevalent, since it 

 demands a degree of local adaptation such as we have else- 

 where tried to show appears to be by no means general. 

 Even if such a process were at work, it is doubtful if it could 

 explain the specific differences observed in the genitalia. The 

 latter could be adaptive in the way described above only if 

 they were an actual impediment to racial crosses, whereas it 



