26 CAUSES OF THE STERILITY [Chap. IX 



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 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 

 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 



