GENERAL INTRODUCTION. xlili 



importance ; tlie destruction of tlie unfit is also important ; 

 but simple variation and the survival of the fittest, unless 

 sterility or prepotency is implied, are not sufficient to 

 account for the origin, from a few simple forms, of a 

 countless number of plants and animals, many of them 

 amazingly complex in organisation. The need of some- 

 thing above and beyond variation and natural selection 

 was especially recognised by the late Mr. Komanes. In 

 what he termed physiological selection he believed he 

 had found the additional factor, physiological selection 

 being something by which isolation was secured, which 

 made sure that some of the varieties so abundantly pro- 

 duced in nature would have a chance of perpetuating 

 themselves, i. e. escape being swamped by intercrossing. 

 In the case of the domestic animals the breeder plays the 

 part of the additional factor ; by isolation he prevents 

 intercrossing. Without fences the famous breeds of cattle 

 that have been so carefully built up would, unless highly 

 prepotent, soon cease to exist as distinct breeds, and 

 would ultimately be merged through intercrossing into 

 as many varieties as there are distinct or isolated areas. 



Hitherto the explanation of one variety persisting 

 while others, equally fit in every way, vanished, has 

 sometimes been that the members of the more fortunate 

 variety, while -fertile with each other, were sterile, or at 

 least less fertile, with other vaineties and with the parent 

 species. It is, of course, conceivable that some sports 

 are sterile except amongst themselves, just as some sports 

 are prepotent, but satisfactory evidence of this is still 

 wanting. The explanation which prepotency aifords does 

 away with the need of sterility, and it does away with the 

 need of rigid isolation, with natural barriers or fences, 

 because the prepotent forms have so much " character " 

 that, however mated, some of the offspring inherit their 

 own structural and other peculiarities. 



What, it may here be asked, do we understand by 

 prepotency and interbreeding ? Any animal, male or 

 female, which strongly impresses its own peculiarities of 

 form, colour, disposition, &c., on its offspring, is pre- 



