Page - 316 - in The Origin of Species

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 agroup 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-