HYBRIDISM 31 



In regard to the sterility of hybrids, in which the 

 sexual elements are imperfectly developed, the case is 

 somewhat different. I have more than once alluded to 

 a large body of facts showing that, when animals and 

 plants are removed from their natural conditions, they 

 are extremely liable to have their reproductive systems 

 seriously affected. This, in fact, is the great bar to the 

 domestication of animals. Between the sterility thus su- 

 perinduced and that of hybrids, there are many points 

 of similarity. In both cases the sterility is independent 

 of general health, and is often accompanied by excess of 

 size or great luxuriance. In both cases the sterility oc- 

 curs in various degrees; in both, the male element is the 

 most liable to be affected; but sometimes the female more 

 than the male. In both, the tendency goes to a certain 

 extent with systematic affinity, for whole groups of ani- 

 mals and plants are rendered impotent by the same un- 

 natural conditions; and whole groups of species tend to 

 produce sterile hybrids. On the other hand, one species 

 in a group will sometimes resist great changes of condi- 

 tions with unimpaired fertility; and certain species in a 

 group will produce unusually fertile hybrids. No one 

 can tell, till he tries, whether any particular animal 

 will breed under confinement, or any exotic plant seed 

 freely under culture; nor can he tell till he tries whether 

 any two species of a genus will produce more or less 

 sterile hybrids. Lastly, when organic beings are placed 

 during several generations under conditions not natural 

 to them, they are extremely liable to vary, which seems 

 to be partly due to their reproductive systems having 

 been specially affected, though in a lesser degree than 

 I when sterility ensues. So it is with hybrids, for their 





