Chap. VIII. CAUSES OF STERILITY. 265 



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 accom- 

 panied 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 ren- 

 dered 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 some- 

 times 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 confine- 

 ment or any 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 ex- 

 tremely liable to vary, which is due, as I believe, 

 to their reproductive systems having been specially 

 affected, though in a lesser degree than when sterility 

 ensues. So it is with hybrids, for hybrids in successive 

 generations are eminently liable to vary, as every expe- 

 rimentalist 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, independently of the general 

 state of health, is affected by sterility 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 



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