THEORETICAL IMPLICATIONS 123 



sudden reassertion of latent ancestral characters in a pure-bred 

 stock, and false reversion or vicinism due to crossing. Descriptively 

 both may be called " reversions," but they differ in their nature 

 and their causes. He also distinguishes reversion to a known 

 ancestor from " systematic atavism " to ancestors which are only 

 reputed to be such on taxonomic grounds. 



We would use the term " reversion" to include all cases where, 

 through inheritance J there reappears in an individual some character 

 or combination of characters which was not expressed in his 

 immediate Hneage, but which had occurred in a remoter but not 

 hypothetical ancestor. We say " through inheritance " in order 

 to exclude those cases where the reappearance can be accounted 

 for in some other way. There is no reason for complicating 

 the idea by calling the reversionary character " abnormal," 

 for abnormality is often difficult to define. 



If we can arrange a series of related types on an inclined plane 

 in order of their evolution, with the most recent highest up, we 

 can imagine the offspring of one of the highest slipping back 

 (as regards one or several of its characters) to a lower level — - 

 slipping back beyond the grade represented by its own family 

 or stock, slipping back out of its species-grade altogether, and 

 so forth. These " throw-backs " might be described as family- 

 reversions, stock-reversions, species-reversions, and so on. 



§ 3. Theoretical Implications 



The general idea behind the term " reversion " is that particular 

 features characteristic of an ancestor may lie dormant — i.e. un- 

 expressed in development — for generations, and may suddenly 

 reassert themselves. 



In the mosaic which composes an inheritance there may be ^ Yw' 



included items of ancient origin which can lie latent generation ^/^'^ 

 after generation, remaining unexpressed in development for 

 lack of the appropriate liberating stimulus, or for other reasons. 

 Certain potentialities or initiatives, which really form part of 



