VARIATIONS OF GARTER-SNAKES. 97 



CONCLUSION. 



If the affinities of the different forms as incHcated above (fig. 34) 

 be accepted, this group, extending as it does from the Mexican 

 plateau to the Great Lakes region, exhibits a progressive decrease 

 in size and scutch ation from the Mexican plateau toward the extreme 

 limits of its range. The reduction is slight in each particular form, 

 but when the extremes are compared with the center it becomes very 

 apparent. Thus, hutleri and megalops in northern Mexico are the 

 extremes in the number of scales in each of the series which I believe 

 to be correlated with size, while at the same time a slight knowledge of 

 these forms is sufficient to show that there is also a striking difference in 

 size between the two. It should also be noted that the areas in which 

 the transitions from one set of scale formulas to another takes place 



radix butleri 



marcianus 



megalops 



Fig. 34.— Phylogenetic development of the Radix group. 



lie mostly between the forms, and are apparently very narrow, 

 except between radix and hntleri since radix shows a marked tend- 

 ency toward a reduced formula throughout the prairie region. 



If the relationships are as I have described them, it should be 

 noted (1) that in each region of different environmental conditions 

 which it occupies the group is now represented by a different form; 

 (2) that each form is characterized by a smaller scutellation than 

 its neighbor toward northern Mexico, and exhibits its minimum 

 scutellation at some point on the outskirts of its range, where it 

 comes in contact with another form with the scutellation similar to 

 that of the reduced individuals; (3) that the most dwarfed repre- 

 sentative of the group is now found in the form farthest removed, 

 geographically and genetically, from the northern part of Mexico. 



