CHAPTER VIII 



REVERSION TO OLD TYPES AND THE MAKING 



OF NEW ONES 



1. The Distinction between Reversion and 



Atavism 



There are two ways in which types of animals or 

 plants that are different from the present ones may 

 be conceived to arise, namely, by the reappearance 

 of old types and by the formation of new ones. In 

 the reappearance of old types a distinction may be 

 drawn between reversion and what has been termed 

 atavism. 



Atavism, or "grandparentism," may be defined as 

 skipping a generation with the result that a particu- 

 lar character in the offspring is unlike the corre- 

 sponding character in either parent, but instead, 

 resembles the character in one of the grandparents. 



In reversion, on the contrary, a character reappears 

 which has not been manifest perhaps for many gen- 

 erations, although it was actually present in some 

 remote ancestor. J. Arthur Thomson's definition 

 of reversion is : "All cases where through inheritance 

 there reappears in an individual some character 

 which was not expressed in his immediate lineage, 

 but which had occurred in a remoter, but not hypo- 

 thetical, ancestor." 



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