370 THE AMERICAN NATURALIST 



[Vol. LVI 



TEEQUENCY TABLE IV 

 ; Number or Vertebra within 



It has generally been taken for granted, as a basic as- 

 sumption, that such differences as those here shown to 

 hold between two successive year-classes, and between 

 successive groups within a single year-class, are indic- 

 ative of racial distinction. Obviously this assumption 

 can not be maintained as wholly true. Moenkhaus (1895, 

 1898) indeed long ago demonstrated the occurrence of a 

 significant annual variation within one race of fishes (in 

 the case of the darters Percina caprodes and Boleosoma 



