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THE AMERICAN NATURALIST [Vol. XLII 



narrow ' ' ; just as has long been known to be the case in mammals 

 and birds, and as Ortmann has recently affirmed to be the rule 

 in crawfishes. 



He considers that 



"Experimental work alone can sufficiently reveal the influence of 

 the environment upon the dwarfing and scutellation of these snakes. 

 In the case of the garter-snakes, however, it should be noted: (1) That 

 most of the forms are the result of dwarfing. (2) That the amount 

 of dwarfing does not seem to be directly associated with the nature of 

 the environment, for the form inhabiting a particular region is only 

 slightly different from its nearest neighbor in the same group, while 

 forms of widely different scutellation may inhabit the same region. 

 Thus the conditions which apparently determine the scutellation of any 

 form is the scutellation of its immediate progenitor, and the dwarfing 



He believes that he is "justified in concluding that the dwarf- 

 ing is associated in some way with the environment." He then 

 cites Allen's law (1876) 2 that the environmental conditions at 

 the center of origin are most favorable for the existence of any 

 group, and says : 



"However this may be, the following facts will stand: (1) That the 

 maximum scutellation and size in the genus Thamnophis occurs at the 

 center of dispersal, and the forms that have been produced in the 

 history of its migration have been formed principally by dwarfing and 

 reduction in scutellation; (2) that the variation in the number of scales 

 in the different series is definite and not promiscuous, and is correlated 

 in a remarkable degree with changes in the environment. The develop- 

 ment of the different groups has thus been orthogenetic." 



He continues: 



" From these facts it seems to me that the most tenable hypothesis 

 of the evolution of the genus Thamnophis is that it originated and 

 became differentiated into four main groups in northern Mexico. From 

 : ' ' In a general way, the correlation of size with geographical distribu- 

 "1. The maximum physical development* of the individual is attained 



"2. The largest species of a group (genus, subfamily, or family, as the 

 case may be) are found where the group to which they severally belong 

 reaches its highest development, or where it has what may be termed its 

 center of distribution. . . . » '—Bull Geol. and Geograph. Surv. Terr., Vol. 

 II, No. 4, p. 310, July 1, 1876. 



