250 A STUDY IN HEREDITY 



Two things are evident from the foregoing. First, that there 

 is on the average a greater tendency towards reversion than 

 towards evolution, that is, there is a greater tendency to revert 

 towards the ancestry than away from it, in other words, there is a 

 greater tendency to let lapse in the ontogeny the last steps made 

 in the phylogeny than to add other steps to them. Secondly, the 

 strength of the tendency towards reversion is proportionate to the 

 swiftness of the antecedent evolution, and, therefore, species which 

 have been quickly evolved, tend to retrogress swiftly, whereas 

 species which have been slowly evolved tend to retrogress slowly. 

 For this reason it is that characters long established in the species 

 are much more stable than more recent characters, for, in the 

 former case, reversion, to be appreciable, must be to an extremely 

 remote ancestor, whereas in the latter, reversion to a much less 

 remote ancestor results in appreciable retrogression. 



Suppose now a certain character in a line of individuals has 

 undergone evolution. Denote by the symbols A, B, C, D, E, F, 

 the evolution of the character in successive individuals of the line, 

 A being the rudimentary character as it appeared in the first of 

 the line who had it, F the character when it reached its highest 

 perfection. Suppose that cessation of selection occurs as regards 

 this character. Then F tends to be lapsed, and, when it is 

 lapsed, E reappears at the end of the ontogeny. But thereafter 

 E also tends to be lapsed, and D to reappear, and so on, till, in 

 the continued absence of selection, at length A reappears. But, 

 under the same law, A tends likewise to disappear, and then the 

 character vanishes utterly, and the race reverts to that ancestral 

 condition when the character did not exist. In this manner, I 

 take it, useless parts disappear absolutely. Thus have 

 disappeared, for instance, the limbs of the snake. Thus have 

 disappeared the eyes of some cave-dwelling animals, and the 

 many useless parts of parasites. Thus have vanished innumerable 

 useless parts in every plant and animal. 



We are now in a position to consider the part played by 

 reversion in nature. Every complex individual, as we know, 

 varies in a thousand ways, great and small, from its parent, but 



