244 A STUDY IN HEREDITY 



vary, they must do so primarily in one of two ways : Either they 

 must revert to the ancestral type, and resemble it more than the 

 parent did, or else they must diverge from it still more than did 

 the parent. The former variation we term "atavistic," the latter 

 we may term "evolutionary," since it is on the lines of these latter 

 variations that evolution proceeds. But of so-called atavistic 

 variations there are also two kinds, one of which is really 

 atavistic and reversionary, whereas the other, though apparently 

 atavistic, is actually evolutionary. True reversion occurs only 

 when the individual varies so from his parent that, in his develop- 

 ment, he does not recapitulate the whole of the life-history of his 

 race, but stops short at a point reached by a more or less remote 

 ancestor, whom in this way he resembles more than he does his 

 parent. False atavism occurs when the individual, at an early 

 stage of his existence, begins by recapitulating the whole of the 

 life-history of his race up to his parent, but during a later stage 

 retraces, or apparently retraces, some of the last steps made by 

 himself in his development and by the race in its evolution, and 

 thus, by a species of evolutionary variation, resembles a more 

 or less rem ate ancestor more than he does the parent. Examples 

 of this false kind of atavism are plentiful in nature. 



The points here set forth are these : First, that development 

 is a recapitulation of evolution ; in other words, that every indi- 

 vidual repeats, though very rapidly and indistinctly, the life-history 

 of his race, beginning with the unicellular organism and ending 

 with the parent. Secondly, that an individual may so vary from 

 his parent that he does not recapitulate the whole of the phylogeny, 

 and that this constitutes true atavism, true reversion. Thirdly, 

 that there is a false atavism, which is really evolutionary. This 

 occurs when an individual, after reaching the full development of 

 his parent, retraces sorhe of the last steps of the ontogeny, and so 

 resembles an ancestor more than he does his parent. More need 

 not be said concerning the first proposition. As regards the third, 

 it has been said above that examples of false atavism are frequent. 

 From the nature of the case observation of it is difficult, for in 

 every individual this retracement of the ontogeny, this false 



