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



more probably lies in some imperfection in the original 

 act of impregnation, causing the embryo to be imper- 

 fectly 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 inde- 

 pendent 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 some- 

 times the female more than the male. In' both, the 

 tendency goes to a certain extent with systematic affin- 

 ity, 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 fer- 

 tility; and certain species in a group will produce un- 

 usually fertile hybrids. No one can tell, till he tries, 

 whether any particular animal will breed under confine- 

 ment, 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, 



