VARIATIONS OF GARTER-SNAKES. 193 



brates. However, any attempt to determine from geographic and 

 variation data that internal factors, the direct influence of environ- 

 mental conditions, or in fact any other cause is responsible for this 

 close correlation of particular forms with particular environments 

 must fail to be conclusive, for experimental work alone can definitely 

 reveal the influence of the environment upon the dwarfing ami scu- 

 tellation in these snakes. In the case of the garter-snakeg, however, 

 it should be noted: 



1. That most of the forms are the result of dwarfing. 



2. That the amount of dwarfuig does not seem to be directly as- 

 sociated with the nature of the environment, for the form inhabiting 

 a particular region is only slightly dift'erent from its nearest neighbor 

 in the same group, while forms of widel}^ dift'erent scutellation may 

 inhabit the same region. Thus the conditions which apparently deter- 

 mine the scutellation of any form is the scutellation of its immediate 

 progenitor, and the dwarfing which it has itself undergone. 



While therefore — although it is without the province of the present 

 work to seek to determine just what factors in the environment 

 make it unfavorable for these snakes — I believe that I am justified 

 in concluding that the dwarfing is associated in some way with the 

 environment; it is as if the cHfi^erent environmental complexes en- 

 countered by the groups as they radiated from the center of dispersal 

 were generally unfavorable, and the most tenable hypothesis is that 

 the changed conditions of environment acted as a stinudus (unfavor- 

 able to growth) upon these snakes (Tower, 1906, 294-296). If 

 this is true, isolation while attending the production of the forms in 

 each group has had no other apparent effect than that of keeping 

 them distinct as they were formed. 



Allen (1876, 310) concludes from such evidence that the environ- 

 mental conditions at the center are the most favorable for the 

 existence of any group, and Adams (1904, 210) that the center of 

 dispersal corresponds to a center of optimum vital conditions. It 

 follows from Adams' standpoint that as a form enters other regions 

 in which there are any changes in the optimum conditions these 

 changes may be considered as unfavorable, whether they are in 

 the way of an increase or decrease, since the zero point of a stimulus 

 corresponds to the vital optimum and is situated between the 

 minimum and maximum, both of which result in death. 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 principall}'^ by dwarfing and b}' a 

 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. 



