136 Heredity. 



ordinary variation is the fact that the descendants of 

 hybrids are more apt to revert than the hybrids them- 

 selves. Darwin says (Variation, p. Go) that this is a 

 general rule. 



Now, whether reversion be due to the sudden excite- 

 ment of a tendency which has long been transmitted in 

 a dormant state by the ova, or whether it is due to the 

 appearance, of a new variation which resembles an old 

 one, we can readily understand how, according to our 

 theory of heredity, crossing should call this power into 

 action. During the evolution of the species each he- 

 reditary peculiarity has been established in the egg by 

 gemmules, and anything which prevents the egg from 

 following its normal course and developing the recently 

 acquired characteristics of the species, would allow older 

 characteristics to appear in their place. 



We know that animals which are very widely sepa- 

 rated are infertile, and we can understand that even 

 when the difference between two species is not great 

 enough to prevent them from crossing, those cells of 

 their bodies which have varied most may be so different 

 from each other that gemmules from the one cannot 

 fertilize the egg-particles which are to produce the other, 

 or when they do fertilize them they may give rise to a 

 variation which is so different from the normal cell that 

 it cannot live. The cells which precede these in the order 

 of growth being less different in the two parents, would 

 be much more favorably situated, and would thus give 

 to the embryo a characteristic of longer standing than 

 the peculiarities of either parent. On the other hand, if 

 reversion is simply variation, we can see that crossing 

 might excite reversion just as it excites variability. 



