VARIATIONS OF GARTER-SNAKES. 187 



2. The supralabials are normally 8 in the two forms in Mexico and 

 7 in the others. The infralabials are normally about 10 in all forms. 



3. The ventrals, subcaudals, and tail length are, so far as known at 

 present, practically the same throughout the entire range of the 

 group. 



Thus the geographic variations in the scutellation consist of a maxi- 

 mum of 19-17 dorsals and 8 supralabials in the form whose principal 

 distribution is on the Mexican plateau (eques), and of a reduction in 

 the dorsal scale formida in the forms to the southward and in the 

 number of supralabials in those to the northward of the range of this 

 form. It only remains to be pointed out that the area of principal 

 reduction between the forms with a different scutellation is small. 



DISCUSSION OF ORIGINS. 



In the general discussion of the characters and variation in the 

 genus, as well as in the detailed discussion of the various forms, I 

 hold to have established two fundamental facts as follows: (1) That 

 the genus ThamnopMs is naturally divided mto four groups of nearly 

 equal importance, the component forms of which are directly related; 

 (2) that each of these groups is represented by a form in the northern 

 part of Mexico and in southwestern United States which has the 

 maximum scutellation for the group. To what extent do these facts 

 throw light upon the question of origins? Darwin pointed out in his 

 Origin of Species that on the principle of descent with modification 

 the only rational view is that the several species of a genus have been 

 derived from a common progenitor and have undergone modification 

 in the course of dispersal. We are then confronted with the problem 

 of locating the center of origin of a. genus before we can intelligently 

 search for laws governing the modifications which have resulted in 

 the component forms as we find them at the time of our study. 



Various criteria have been formulated toward this end.*^ At least 

 one of these can not be used in this instance, as it is based on peculiar 

 conditions existing in one group of animals: it is the direction of the 

 annual migration routes of birds (Palmen). Of the others I believe 

 that Adams's sixth criterion is of the most general and easy appli- 

 cation as well as logically (assuming, of cours(>, the theory of descent 

 and single centers of origin) the most reliable. This rule is that the 

 different lines of dispersal in any genus radiate from, or, conversely, 

 converge toward, the geographical center of origin. As already men- 

 tioned, I believe that this rule follows logically from the theory of 

 descent formulated by Darwin, so that if only the lines of dispersal 

 can be determined in any group in which there are more than one, 

 it should be relatively easy to fix upon the center of origin of that 

 genus. In discovering that different nearly equal groups of related 



a See Adams (1902 b). 



