194 BULLETIN «!, UNITED STATES NATIONAL MUSEUM. 



The development of the (hU'erent groups has thus l:)een orthogenetic. 

 This is nowhere better shown than in such forms as radix, where, 

 as one passes from tlie western to the eastern parts of the range, 

 not only the average but also the maximum and minimum num- 

 bers of plates in the different series decrease steadih^, until the 

 range of hutleri is reached; and that this is in some way associated 

 with the environment is shown in the numerous instances in which 

 the transition from the scutellation of one form to that of another 

 takes place rapidly, and along the boundary of different sets of envir- 

 onmental conditions. Even in the latter instances the variations are 

 evidently orthogenetic, as shown by the fact that geographically 

 there are marked difl'erences in the maximum, minimum, and aver- 

 age number of scales in the different series. I have failed to find the 

 slightest evidence that forms have been produced by mutations in 

 this genvis. 



From these facts, then, it seems to me that the most tenable 

 hypothesis of the evolution of the genus Thamnopliis is that it orig- 

 inated and became differentiated into four main groups in northern 

 Mexico. From this region the groups radiated in all directions, but 

 principally to the northward, and wherever they entered different 

 regions the changed environmental conditions acted as an unfavor- 

 able stimulus, wliich retarded growth, and differentiated the groups 

 into dwarfed forms. If this explanation is the true one, the influence 

 of the environment without doubt aff'ects the young snake before 

 birth, since the scutellation, at least as regards the number of scales 

 in the dorsal, labial, ventral, and subcaudal series, is determined 

 before the young are born. 



THE GARTER-SNAKES AS MATERIAL FOR EXPERIMENTAL INVESTIGATION. 



So far as I know, snakes have rarely been usetl for experimental 

 purposes. They are unfitted for this both by the fact that apparently 

 but one brood is raised each year and that they are difficult to keep 

 in captivity. The garter-snakes, however, are free from both of these 

 objections, since they are hardy in captivity, and while raising but one 

 brood a year are very prolific. For these reasons they should present 

 very good material for the investigator, the more so as they may 

 be had in numbers almost anywhere in the United States. It should 

 be borne in mind, however, that when in captivity they must be 

 kept, as nearly as possible, in natural conditions. 



Undoubtedly the problems that should first be attacked are the 

 inheritabiljty of the scale characters and the influence of difl'erent 

 factors (inbreeding, unfavorable conditions of food and temperature) 

 in producing dwarfing. The latter problem Jnay be attacked in two 

 ways: An attempt may be made to cause an increase in size and 

 scutellation in dwarfed forms, such as hutleri and ordinoides, or to 



