26 CAUSES OP THE STERILITY [Chap. IX. 



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 unnatural crossing of two species, 

 the reproductive system, independently 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 in- 

 appreciable by us; in the other case, or that of hybrids, 

 the external conditions have remained the same, but 

 the organisation has been disturbed 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 compounded 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 off- 

 spring from generation to generation the same com- 

 pounded organisation, and hence we need not be sur- 

 prised that their sterility, though in some degree varia- 

 ble, 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 hybrids being caused by two constitutions being com- 

 pounded into one has been strongly maintained by Max 

 Wichura. 



