Tlie Evidence from Hytrids. 137 



Summary of Chapter. 



The study of hybrids gives us a means of comparing, 

 within certain narrow limits, the parts which the two 

 sexual elements play in heredity. The influence of each 

 sex can, in a certain sense, be studied by itself when a 

 given species is used in the one case as the father of a 

 hybrid, and in another case as the mother. The value of 

 crossing as an experiment in heredity is greatly limited, 

 however, by the fact that, although we can study the in- 

 fluence of one sexual element unobscured by the other 

 element from the same species, it is obscured and compli- 

 cated by the influence of this element from an allied 

 species, and in all organisms which can breed together 

 the reproductive elements must be essentially alike. 



Hybrids do, however, present a number of peculiarities 

 which agree perfectly with what we should expect ac- 

 cording to our hypothesis, and certain of these are inex- 

 plicable without it. 



Hybrids and mongrels are highly variable, as we 

 should expect to be. the case, according to Darwin's 

 pangenesis hypothesis. This hypothesis fails to account 

 for the fact that hybrids from forms which have long 

 been domesticated are more variable than those from 

 wild species or varieties, or for the very remarkable fact 

 that the children of hybrids are much more variable than 

 the hybrids themselves. 



Our theory not only explains the variability of hy- 

 brids, but it also accounts for the two latter peculiarities, 

 for crossing will not give rise to a marked or conspicuous 

 variation unless the hybrid inherits numbers of gem- 

 mules, and as domesticated animals and plants live 

 under unnatural conditions they are more favorably 

 placed than wild forms for the production of gemmules. 



