CHAPTER IX 



OLD TYPES AND NEW 



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 particular 

 character in the offspring is unlike the corresponding 

 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 hypothetical, ancestor." 



This distinction between atavism and reversion be- 

 comes clearer by illustration. 



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