OLD TYPES AND NEW 149 



for the ancestral character or characters in this type 

 of hereditary behavior, as said before, reappear only 

 after a lapse of many generations. 



2. FALSE REVERSION 



"Around the term ' reversion,' " Bateson observes, 

 "a singular set of false ideas have gathered them- 

 selves . " In proof of this statement there may be cited 

 at least five categories of apparent reversion which 

 properly ought not to be classed as true reversion. 



a. Arrested Development 



Feeble-mindedness is not reversion to ancestral 

 forms of less intelligence, but an instance of arrested 

 development when, for some reason, the individual 

 fails to accomplish his normal cycle of development. 



Likewise harelip in man is not a case of reversion 

 to rabbit-like ancestors in which harelip is the nor- 

 mal condition, but it is ordinarily due to an arrest or 

 failure of certain embryonic steps that are essential 

 to the development of the usual form of human lip. 



b. Vestigial Structures 



These are the vanishing remains of characters that 

 were formerly of significance. They do not represent 

 something latent that is now reappearing, for they 

 have never yet disappeared phylogenetically, and con- 

 sequently they cannot be regarded as true reversions. 



The muscles under the scalp which enable those 

 persons possessing them to wiggle the ears; the 

 palatine ridges in the roof of the mouth of many 



