16 ELEVENTH REPORT. 



aon-iitilitarian, we apparently have in the garter-snakes a case of evolu- 

 tion along fixed lines as the result of definite variation. Even if we could 

 admit the selection of fluctuating variations as the dominant factor in the 

 evolution of these forms, we should encounter the additional difficulty that 

 it (selection) is apparently unable to create a new species, the form slipping 

 back to the original condition when the selection ceases. The nature of the 

 variations also seems to me to debar De Vries's theory of mutations as a 

 possible explanation, for, according to De Vries, the specific changes are 

 sudden and fixed, the neAV race appearing fully formed, which is certainly 

 not the case in these snakes, unless we consider the new race as having been 

 formed by small successive mutations of the same kind, which would, if I 

 understand him rightly, be contrary to De Vries's idea of mutations, for he 

 says that they "take place so far as experience goes without definite direc- 

 tion." We have apparently in these snakes an example of definite but 

 gradual variation, in that there is in each group a gradual development of 

 forms along a fixed path, without the aid of natural selection. 



A theory that accounts for the definite evolution of this genus without 

 the aid of selection is that there has been in each group a gradual modifica- 

 tion of the forms under the influence of the environment. The determinate 

 variation of the average type, the close association of the forms with different 

 geographic regions and the consequent correspondence of areas of transition 

 in characters with intermediate geographical conditions, would seem to 

 render this conclusion unavoidable, if, as seems evident, selection is not 

 operative here. It should he noted, however, that, while the evolution of 

 the genus has been distinctly orthogenetic and associated with the environ- 

 ment, it does not appear that the latter has, as has often been supposed 

 ]')}' adherents of this theory, a specific effect. This would be difficult to de- 

 termine if we were dealing with one group, for example one that pushed 

 northward from the Mexican plateau, for here there would be an increasing 

 difference in climatic conditions associated with accumulated modifications, 

 Ijetween the range occupied by the form at the center and that of the' most 

 outlying species. But when all four groups are considered it is seen that 

 the same modifications appear whether the group is pushing into the .tropic 

 or temperate regions, or into deserts, semi-arid plains, or humid forests, 

 and it is difficult to conceive of environmental conditions common to all 

 of the regions occupied l)y these snakes that would exert such a specific 

 effect upon them. It seems rather that throughout the genus the germ cells 

 have such a restricted number of potential responses that the different 

 groups have tended to vary in the same direction (homoplasy) under the 

 influence of the environments which have been encountered; that is to say, 

 the similarity of the response is conditioned by the constitution of the animal, 

 the environment only acting as a stimulus upon the germ cells. 



This theory would seem to satisfactorily explain why some directly related 

 forms intergrade while others do not. for if infertility is in proportion to 

 j^hysiological diversity, as Darwin held, it is ciuite evident that as the new 

 form produced as a group pushes into a new region becomes more or less modi- 

 fied, it will also probably become physiologically different from the parent 

 stock and be more or less unable to cross back. This is essentially Elmer's 

 Genepistasis, or Entwicklungstillstand, the standing still of certain forms at 

 definite stages in development while others continue. But it should l^e noted 

 in this case that the forms that progress each time are always associated 

 with new geographic regions, and clo not occur, as Eimer holds that they can, 

 in the same region with the parent stock. 



