252 A STUDY IN HEREDITY 



when they had not come into existence. Others, in which 

 reversion is yet incomplete, still persist, and are known to us as 

 vestigial remains. It should, however, be noted that, when a 

 vestigial structure is more developed earlier in the ontogeny than 

 it is later, this indicates that its retrogression is due not only to 

 reversion, the result of true atavism, but also to false rever- 

 sion, the result of reversed selection. Such a structure must 

 have become not only useless, but worse than useless, during the 

 phylogeny. 



Every complex animal, therefore, in the successive stages of 

 its development does not represent exactly successive stages in 

 the evolution of its race. At each stage of the ontogeny are 

 present useless structures, which have retrogressed backwards 

 towards a more ancient order of things ; and at every stage of 

 the ontogeny structures are absent, which were present in the 

 phylogeny because they were useful, but which since underwent 

 complete retrogression, because they subsequently became useless. 

 Here, then, we have the explanation of the fact that ontogeny is 

 only a very vague recapitulation of the phylogeny. Doubtless, if 

 a high animal, a man for instance, lived during his ontogeny in 

 a succession of environments similar to those in which his race 

 was evolved, his ontogeny would more exactly recapitulate the 

 phylogeny than it actually does, for, in that case, structures, which 

 had been useful during the phylogeny, would continue to be so 

 during the ontogeny, and so would be preserved. But consider 

 how vastly different is the environment in which the embryo of 

 man develops, from the environments in which his race evolved. 

 The embryo develops in the uterus, but its free prototypes 

 struggled each for itself in a world full of enemies, full of 

 eliminating agencies. How many parts, therefore, have become 

 useless to the embryo, which were useful to the prototypes. 

 How vast is the field in which retrogression has worked. Is it 

 any wonder, then, that the ontogeny of man is only a vague re- 

 capitulation of his phylogeny. Reversion, then, is the necessary 

 complement of evolution, and without it there could be no 

 evolution, except of the simplest kind. Without reversion there 



