CHAPTER VII. 

 ATAVISM (continued). 



Supernumerary Digits, Limbs, and Mammary Glands. 

 Nothing illustrates so forcibly the necessity of critically 

 examining suspected cases of atavism as those abnor- 

 malities collectively known as Polydactyly. At the 

 outset it may be stated, contrary to the prevailing 

 opinion, that supernumerary digits are very rarely 

 atavistic. 



The various examples of extra fingers, toes, arms, and 

 legs, described in the chapter on Dichotomy, serve to 

 show that if all supernumerary parts are to be regarded 

 as reversions we must find vertebrates provided with an 

 unlimited number of toes, double hands and feet, and 

 more than two pairs of limbs. 



No vertebrate animals other than fish and the Ichthyo- 

 saurii, possess on each limb more than five digits, 

 therefore when the number of toes or fingers exceed on 

 each limb this typical number, it must if we, with 

 Darwin, regard the accessory digit as atavistic, be a 

 reversion to an Ichthosaurian or a fish form. The 

 distance is far too great, and in doing so we violate the 

 rule that atavistic parts do not belong to forms palae- 

 ontologically remote or systematically far distant. 



