238 ON THE ORIGIN OF SPECIES 



natural 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 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 experi- 

 mentalist 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 

 be inappreciable 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 

 different structures and constitutions 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 relation of the different 



f>arts and organs one to another, or to the conditions of 

 ife. When hybrids are able to breed inter se, they 

 transmit to their offspring from generation to gene- 

 ration the same compounded organisation, and hence 

 we need not be surprised that their sterility, though in 

 some degree variable, rarely diminishes. 



It must, however, be confessed that we cannot under- 



