VARIATIONS OF GARTER-SNAKES. 191 



forms may originate in favorable environments in arid regions, just 

 as strictly aquatic forms may do so. I believe, therefore, that, 

 in cases of this kind at least, these two criteria (abundance and least 

 restriction to a particular habitat) are not valid, for a group may 

 originate in a region where the environmental conditions to which 

 it is adapted are more restricted than elsewhere. 



The fifth criterion of Adams, "Location of greatest productiveness 

 and its greatest stabihty" (Hyde, 1898, p. 575), is in the main inap- 

 plicable in tliis study, owing to our almost total ignorance of the 

 breeding habits of the various forms. It seems worth recording, 

 however, that the most dwarfed forms (those with the smallest 

 scutellation) have apparently a fewer number of young in a brood 

 than do the larger forms, as might be expected. For example, 

 hutleri generally has about a dozen young in a brood, while radix may 

 have forty or fifty. As the relative productiveness thus seems to be 

 correlated with size in this genus, the location of the area of greatest 

 productiveness would be near the center of origin, but even if tliis 

 should be found to be the case it would prove only that the test can 

 be used in genera in which the forms are the result of dwarfing. 

 However, as said above, so little is known of the breeding habits of 

 these snakes, that it is hazardous to attempt any general conclusions 

 at this time. 



Another criterion that is quite easy to apply is the one formulated 

 by Allen in 1877 (1877, 378) : ''The representatives of a given species 

 increase in size toward its hypothetical center of distribution, wliich 

 is in most cases doubtless also its original center of dispersal." In 

 snakes it is doubtful if we have any such a thing as "adult size" in 

 the sense employed in birds and mammals, so that, without some 

 method of indicating dwarfing, tliis criterion is valueless. Fortu- 

 nately we believe that differences in the number of scales in the dorsal, 

 labial, and ventral series is, as outlined in the chapter on variation, 

 an index to the relative size in these snakes, and, if so, we have in 

 these characters a ready means of determining the relative dwarfing 

 of the different forms, even if young specimens alone are available 

 for study. 



Applying this criterion to the conditions outlined in the description 

 of the different forms the results are startling, for in every group the 

 maximum scutellation is found in the form which inhahits northern 

 Mexico, while from this region the scutellation decreases progressively 

 in the different forms, so that the minimum is only found in the forms 

 which constitute the extreme ends of the series, and are thus genetically 

 and geographically most distant from the form with, the maximum number 

 {at the center) . In other words, the forms of the same group are progress- 

 ively 7nore dwarfed away from northern Mexico. [This does not include 

 those forms of the Sirtalis group (concinnus and sirtalis), which are 



