OF FIRST CROSSES AND OF UTBRrDS. 295 



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 ani- 

 mals. 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 m.ale element is the most liable to be affected; 

 but sometimes the female more than the male. In both, 

 the tondency goes to a certain extent with systematic afii- 

 nity, 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 partic- 

 ular 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 when 

 sterility ensues. So it is with hybrids, for their offspring 

 in successive generations are eminently liable to vary, as 

 every experimentalist has observed. 



Thus we see that when organic beings are placed under 

 new and unnatural conditions, and when hybrids are pro- 

 duced by the unnatural crossing of two species, the repro- 

 ductive system, independently of the general state of 

 health, is affected in a very similar manner. In the one 

 case, the conditions of hfe have been disturbed, tliough 

 often in so slight a degree as to be inappreciable by us; in 

 the other case, or that of hybrids, the external conditions 

 have remained the same, but the organization has been dis- 

 turbed by two distinct structures and constitutions, includ- 

 ing of course the reproductive systems, having been 

 blended into one. For it is scarcely ^ possible that two 

 organizations should be compounded into one, without 



