402 NATURAL SCIENCE [December 



since the retracement on which Reversed Selection works is 

 apparently always small in amount, it never seems to occur in 

 species that have been so rapidly evolved as these garden plants. 

 Their reversion, therefore, seems to be invariably due to true 

 atavism, there being apparently no room for Reversed Selection. 

 Here, then, is a strong proof, convincing proof as it seems to me, 

 that true atavism means a lapsing for good and all of the last steps 

 made in the phylogeny. 



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 phylo- 

 geny 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 lias 

 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 per- 

 fection. 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 do 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 only here 

 and there is a variation useful. The useful variations, in proportion 

 to their usefulness, are preserved and, in succeeding generations, are 



