2.')0 CAUSES OF THE STERILITY [Chap. IX. 



the egg or seed produced by the mother, be exposed to conditions 

 in some degree unsuitable, and consequently be liable to perish at 

 an early period; more especially as all very young beings are 

 eminently 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 im- 

 perfectly 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 sys- 

 tematic 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 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 degcee 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 produced by the 

 unnatural crossing of two species, the reproductive system, inde- 

 pendently of the general state of health, is affected in a very 

 similar manner. In the one case, the conditions of life have been 

 disturbed, though often in so slight a degree as to be inappreciable 



