( cv ) 



received from and through its lespective parent, and must 

 therefore be of inconceivable complexity. We can only speak 

 in generalities about processes of which so little is known, 

 but we cannot be wrong in assuming that sterility is some- 

 times due to the fact that the complex ai^chitecture of one 

 part-nucleus fails in some way to suit the equally complex 

 structure of the other. The individuals of an inter-breed- 

 ing community form a biological whole, in which selection 

 inevitably keeps up a high standai'd of mutual compati- 

 bility between the sexual nuclei. Individuals whose sexual 

 nuclei possess a structure which leads to sterile combinations 

 with those of other individuals are excluded from contributing 

 to the generations of the future. As soon, however, as a 

 group of individuals ceases, from any reason, to breed with 

 the rest of the species, there is no reason why the compati- 

 bility of the sexual nuclei of the two sets should be retained. 

 Within each set, selection would work as before and keep 

 up a high standard of compatibility ; between the sets, com- 

 patibility would only persist as a heritage of past selection, 

 gi-adually diminishing as slight changes of structure in either 

 "or both of the sets rendered them less and less fitted to 

 produce fertile combinations.* 



It is probable that of all the nice adjustments required in 

 the living organism, the mutual adjustment of these incon- 

 ceivably complex part-nuclei is the most delicate and precise. 

 Now, delicately adjusted organs, such as those of sight, i-apidly 

 become incapable of performing tlieir functions when in any 

 species they have been withdrawn from the operation of 

 natural selection ; similarly it is suggested, that the adjustment 

 of sexual nuclei to each other would sooner or later give way 



* I must guard against the inference that the only explanation of 

 sterility is here set forth. It is indeed maintained that incompatibihty 

 of the sexual part-nuclei is the inevitable outcome of enduring asyngaray, 

 and is the almost certain cause of the sterility of hybrids. And it may 

 be suggested that sterility is a result of the combination of two incom- 

 patible germ-plasms in the .«exual cells of the hybrid. When the 

 incompatibility is not strongly marked we can understand how such 

 sexual cells may be capable of fertile fusion with the cells of either 

 parent, but not with those of anotlier h}brid. 



But short of these ultimate effects it must not be forgotten that there 

 are many obscure factors of asyngamy — causes of various kinds which 

 interfere with the fusion under normal conditions or entirely prevent the 

 meeting of the sexual cells. 



