316 ORIGIN OF SPECIES 



elements are imperfectly developed, the case is somewhat dif- 

 ferent. I have more than once alluded to a large body of 

 facts showing that, when animals and plants are removed 

 from their natural conditions, they are extremely liable to 

 have their reproductive systems seriously affected. This, in 

 fact, is the great bar to the domestication of animals. Be- 

 tween the sterility thus superinduced and that of hybrids, 

 there are many points of similarity. In both cases the steril- 

 ity 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 con- 

 ditions; 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 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 genera- 

 tions 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 af- 

 fected in a very similar manner. In the one case, the condi- 

 tions 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 dis- 



