ORGANIC EVOLUTION — PHYSICAL 47 



of the first and euding with those of the last, and there- 

 fore in his development he presents a fleeting resem- 

 blance to each ancestor in ttirn. 



It follows, tlierefore^ that what is known as atavism 

 is nothing other than a failure to recapitulate in the 

 ontogeny the last stages of the pliylogeny ; i. c. it is an 

 arrest of development, the individual halting at a stage 

 reached by a remote ancestor, and developing no 

 farther. A race may differ from its ancestry in three 

 ways : (1) through evolution as a result of selection ; 

 (2) through evolution as a result of reversed selection ; 

 and (3) through retrogression occurring in the absence 

 of selection. As regards traits evolved under the 

 influence of selection, atavism is a simple arrest of 

 development ; that is, the ancestral form is approxim- 

 ated to, because the last stages of the pliylogeny are 

 omitted in the ontogeny. As regards traits evolved 

 under the influence of reversed selection, atavism is 

 also an arrest of development ; the ancestral form 

 reappears because there is an omission in the last 

 stages of the ontogeny to retrace steps previously made, 

 as was done in the pliylogeny. As regards traits sup- 

 pressed through retrogression, i. c. through a lapsing by 

 the race of the last steps of the evolution, there can of 

 course be no atavism ; for instance, if the lost toes of 

 the horse have disa^Dpeared through retrogression, i. e. 

 through a return, in the absence of selection, to a very 

 remote ancestral condition when they did not exist, 

 then their reappearance in the modern horse would be 

 an instance of evolution, not of retrogression, since it 

 would be a return from a more ancient to a much more 

 modern condition ; on the other hand, if the toes have 

 disappeared as a result of reversed selection, the embryo 

 should exhibit them, and they should be present in 

 cases of atavism. 



It is now easy to understand why all races tend to 



