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. 



" Reversion," Prof. Bateson points out, " occurs when the sum- 

 total of the factors returns to that which it has been in some original 

 type. Such a return may be brought about by the omission of an 

 element or elements, or by the addition of some missing element 

 needed to complete the original type. Reversion on crossing is thus 

 the particular case in which one or more missing factors are brought 

 in by the parents of the cross-bred." This is the Mendelian inter- 

 pretation of reversion, and Mr. Bateson does not believe that there 

 are any reversionary phenomena which do not admit of this inter- 

 pretation. 



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

 through inheritance, there reappears in an individual some character 

 or combination of characters which was not expressed in his immedi- 

 ate lineage, 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 re- 

 versionary 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. 



