Chap. IX.] OF FIRST CROSSES AND OF HYBRIDS: 25 



are emiuently sensitive to injurious or unnatural 

 conditions of life. But after all, the cause more 

 probably lies in some imperfection in the original act of 

 impregnation, causing the embryo to be imperfectly 

 developed, rather than in the conditions to which it is 

 subsequently exposed. 



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 superinduced 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 occurs 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 animals and plants are 

 rendered impotent by the same unnatural 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 conditions 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 tiU he tries, 

 whether any two species of a genus will produce more 



