234 CAUSES OF THE STERILITY [CHAP. IX. 



systems seriously affected. This, in fact, is the great bar to the 

 domestication of animals. Between the sterility thus superin- 

 duced 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 condi- 

 tions ; 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 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 produced by the un- 

 natural crossing of two species, the reproductive system, indepen- 

 dently of the general state of health, is affected in a very similar 

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

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

 by us ; in the other case, or that of hybrids, the external condi- 

 tions have remained the same, but the organisation has been dis- 

 turbed by two distinct structures and constitutions, including of 

 course the reproductive systems, having been blended into one. 

 For it is scarcely possible that two organisations should be com- 

 pounded into one, without some disturbance occurring in the 

 development, or periodical action, or mutual relations of the 

 different parts and organs one to another or to the conditions of 

 life. When hybrids are able to breed inter se, they transmit to 

 their offspring from generation to generation the same compounded 

 organisation, and hence we need not be surprised that their sterility, 

 though in some degree variable, does not diminish ; it is even apt 

 to increase, this being generally the result, as before explained, of 

 too close interbreeding. The above view of the sterility of hybrid* 

 being caused by two constitutions being compounded into one has 

 been strongly maintained by Max Wichura. 



