632 Mutations 



upon the systematic affinity between the deriva- 

 tive species and its nearest probable allies. 

 Such reversions are now to be examined at some 

 length and may be adequately treated under the 

 head of systematic atavism. To this form of 

 atavism pertain, on the basis of our definition, 

 those phenomena by which species assume one 

 or more characters of allies, from which they 

 are understood to have descended by the loss of 

 the character under discussion. The phenom- 

 ena themselves consist in the production of 

 anomalies and varieties, and as the genetic 

 relation of the latter is often hardly beyond 

 doubt, the anomalies seem to afford the best in- 

 stances for the study of systematic atavism. 

 This study has for its chief aim the demonstra- 

 tion of the presence of the latent characters, and 

 to show that they return to activity suddenly 

 and not by a slow and gradual recovery of the 

 former features. It supports the assertion 

 that the visible elementary characters are es- 

 sentially an external display of qualities carried 

 by the bearers of heredity, and that these 

 bearers are separate entities, which may be 

 mingled together, but are not fused into a 

 chaotic primitive life-substance. Systematic 

 atavism by this means leads us to a closer ex- 

 amination of the internal and concealed causes, 

 which rule the affinities and divergencies of 



