ASYNGAMY LEADS TO STERILITY 81 



of one part- nucleus fails in some way to suit the equally 

 complex structure of the other. The individuals of an 

 interbreeding community form a biological whole, in which 

 selection inevitably keeps up a high standard of mutual 

 compatibility 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 compatibility 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, 

 gradually diminishing as slight changes of" structure in 

 either or both of the sets rendered them less and less 

 fitted to produce fertile combinations. 1 



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, rapidly become incapable of performing their 

 functions when in any species they have been withdrawn 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 when no longer 

 sustained by selection. If, then, mutual fertility be the 

 result of unceasing selection, and mutual sterility the 



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

 sterility is here set forth. It is indeed maintained that incompatibility of 

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

 and is the probable cause of the sterility of hybrids. Thus it may be 

 suggested that sterility is a result of the combination of two incompatible 

 germ-plasms in the sexual 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 another hybrid. 



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. 



