192 BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



based only on color characters.] The results are exactly those which 

 we should expect if the criterion is a valid one, and the center of origin 

 is in the northern Mexico, as indicated by the convergence of the 

 lines of dispersal, but it must be recognized that while these facts 

 may be in harmony with the theory, they can not be considered as 

 proving it until the cause of dwarfing is known. 



METHOD OF EVOLUTION OF THE FORMS. 



It is not the purpose of this paper to discuss the causes of evolution 

 in the garter-snakes, for the study of distribution and variation alone 

 can not encroach upon the field of experimental morphology in deter- 

 mining the causes and effects of the processes which effect the animal 

 form. It is the province of such studies, however, not only to point 

 out probable origins and relationships, but also to search for the 

 methods of evolution, so that the experimentor may work intelli- 

 gently in his search for the processes which cause the production of 

 one species from another. 



If the range of the forms in the different groups of garter-snakes be 

 carefully examined it will be found (1) that the different forms of the 

 same group are found in different geographical regions, characterized 

 by different environmental conditions; (2) that the area along the 

 common boundary of two forms of the same group, where transition 

 in characters takes place, is relative narrow. 



These conditions are exactly in harmony with Stone's (1903, 659) 

 statement for terrestrial vertebrates that "we never find two geo- 

 graphic races or subspecies of the same form occurring together, 

 except during times of migration," and Jordan's (1905, 547) statement 

 that "given any species in any region, the nearest related species is 

 not likely to be found in the same region nor in a remote region, but 

 in a neighboring district separated from the first by a barrier of some 

 sort." Steere (1894, 419) evidently had the the same point in mind 

 when he formulated the law ''the genus is represented by only a 

 single species in one place," but he expressed it too generally, for as 

 a matter of fact, as shown in this genus, two or more species may 

 occur together in the same environment, but the direct relative of 

 any species is to be found not in the same but in a neighboring envi- 

 ronment. In the light of the results of this investigation, as well 

 as the known instances in which different species occur in the same 

 geographic area but in difi'erent environments, it seems expedient to 

 reformulate the law to read as follows : Directly related forms on any 

 line of descent generally occupy neighhoring environments. Although 

 expressed in different terms, this is exactly the same conclusion 

 arrived at by Ortmann (1906, 512) in his study of the crawfishes of 

 Pennsylvania. 



At the present time it seems to be becoming recognized that the 

 above law is of wide application among the higher orders of verte- 



