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THE AMERICAN NATURALIST [Vol. XLIX 



enormous period. 10 It is clearly distinguishable in the 

 European Pliocene where three species of canids are 

 referred to the genus Vulpes by Schlosser and others. 

 Again, in the Upper Pliocene of India there occurs a spe- 

 cies of fox as well as in the Pliocene of Chinr. In North 

 America the fox is first recorded from the Pleistocene 

 definitely, although an Upper Miocene species (Canis 

 vafer Leidy) is regarded by some as the forerunner of 

 Vulpes and by others as a pro-Vulpes genus. It is there- 

 fore probable that the phylum of the fox diverged from 

 that of the wolf as early as Miocene times, perhaps a 

 million years ago, although the generic distinctions of 

 proportion-characters were not fully acquired until Plio- 

 cene times. The ancient geologic separation of Canis and 

 Vulpes is further indicated by the fact that they do not 

 interbreed. The marked divergence in proportions— the 

 fox small, slender, narrow-headed, a small-mammal and 

 bird catcher, the wolf relatively large, massive, broad- 

 headed, a large-mammal catcher — is accompanied by the 

 gain or loss of several relatively obscure numerical char- 

 acters, such as the cuspules on the incisors and premolars 

 (Fig. 1. r, r, r), which are strong in the wolf, intermediate 

 in the arctic fox, and absent in the red fox. It would 

 appear that the wolf had developed these numerical cusp 

 characters more rapidly than its congeners. In a fossil 

 series the development of sucli cusp characters may be 

 followed stage by stage. 



2. Observations by a Field Zoologist on the Modes of 

 Origin of Numerical and Proportional Characters 

 The special features of the field work developed under 

 C. Hart Merriam's direction in the U. S. Biological Sur- 

 vey are: (1) the vast quantity of comparative material 

 brought under examination, (2) the exact geographic, cli- 

 matic and environmental records, (3) the assemblage of 

 numerous intergradations between species and sub-spe- 

 cies, (4) the precision of the measurements and observa- 



w I am indebted to the authority of Dr. W. D. Matthew for these remarks. 



