39 S NATURAL SCIENCE [December 



repeats, though very rapidly and indistinctly, the life-history of his 

 race, beginning with the unicellular organism and ending, in many 

 cases, 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 evolution. 

 This occurs when an individual after reaching the full development 

 of his parent retraces some 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 atavism, 

 must be very slight — so slight as usually to be inappreciable. There- 

 fore it is only by observing the retracement, not in an individual 

 but in a line of individuals, that it becomes plainly noticeable. It 

 is by taking advantage of such retracement that 'Eeversed Selection,' 

 as it has been termed, eliminates a structure which a change of 

 environment has rendered not only useless, but worse than useless, 

 more rapidly than would otherwise occur under the mere absence of 

 selection. For example, Natural Selection has resulted in the evolu- 

 tion of eyes. In animals dwelling in absolute darkness, e.g. certain 

 cave-dwellers, the eye has become not only useless, but worse than 

 useless, since it is an extremely prominent and tender, and therefore 

 vulnerable, part of the organism. In some such animals we observe 

 that the eye is better developed in the embryo than in the adult. 

 Clearly here the animal in its ontogeny retraces some of the steps it 

 has already made. Clearly also, if ontogeny be a recapitulation of 

 phylogeny, such retracement was made in the phylogeny as well. It 

 follows that when a structure, useless both to the embryo and the 

 adult, is better represented in the former than in the latter, it must 

 have undergone retrogression through the action of reversed selection, 

 and that during the phylogeny, after being useful, it became not only 

 useless, but worse than useless. 



The second proposition, that an individual may so vary from his 

 parent as not to recapitulate the latter stages of the phylogeny, and 

 that this constitutes true atavism, is the main proposition of the 

 present thesis ; but I have yet to prove that this atavism is the cause 

 of true retrogression. 



True atavism can seldom be observed in such of the higher 

 animals and plants as have been evolved under Natural Selection, not 

 because it does not occur, but simply because it is usually masked 

 and slight. It is masked because such complex beings seldom or 

 never retrogress in all their characters at once, and, therefore, such 

 reversion as may occur in this or that particular is associated with 



