The Evidence from Variation. 161 



Summary of Last Two Chapters. 



The study of hybrids and of variation has led to the 

 discovery of a great number of general laws, all of which 

 are perfectly explicable by the theory of heredity, and 

 are precisely what it would lead us to look for, although 

 most of them are absolutely inexplicable without it, and 

 have no place in any other hypothesis which has ever 

 been proposed to account for the phenomena of hered- 

 ity. 



The study of hybrids gives us a means of analyzing to 

 a certain extent the influence of each sex in heredity, 

 but our experiments in this direction are limited b^ the 

 fact that organisms must be very closely related in order 

 to breed together, and parents which are very closely re- 

 lated must be essentially alike in everything except the 

 most recently acquired modifications. So far as they 

 enable us to analyze the influences of the sexes, the re- 

 sults furnished by hybrids agree with the demands of 

 our theory. This furnishes an explanation of the great 

 variability of hybrids, as compared with the pure parents, 

 and it also enables us to understand why hybrids from 

 domestic races should be more variable than those from 

 wild races. 



The remarkable fact that the descendants of hybrids 

 are more variable than the hybrids themselves receives a 

 simple explanation by our assumption that exposure of 

 the various cells of the body to unnatural conditions is 

 the prime cause of variability, and that it acts indirectly 

 by causing the production of gemmules. 



Some of the recorded facts regarding hybrids are so 

 very peculiar that it would be difficult to devise better 

 tests than they furnish of the truth of our theory. 

 What could be more curious or more opposed to the 



