408 



THE AMERICAN NATURALIST [Vol. XLIII 



by 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 physiological 

 diversity, as Darwin held, it is quite evident that as the 

 new form produced as a group pushes into a new region 

 becomes more or less modified, it will also probably be- 

 come physiologically different from the parent stock and 

 be more or less unable to cross back. This is essentially 

 Eimer's Genepistasis, or Entwicklungstillstand, the 

 standing still of certain forms at definite stages in de- 

 velopment while others continue. But it should be noted 

 in this case that the forms that progress each time are 

 always associated with new geographic regions, and do 

 not occur, as Eimer holds that they can, in the same re- 

 gion with the parent stock. 



In conclusion I would like to point out that the purpose 

 of this paper is to describe the method of evolution in the 

 genus Thamnophis rather than to discuss the cause of the 

 variations. It is evident that the suggested relation be- 

 tween the action of the environment and the nature of the 

 response of the organism can only be tested by experimen- 

 tation. I may add further that the interpretation offered 

 of the conditions in this genus depends upon whether or 

 not the relationships of the forms and the lines of de- 

 velopment are as outlined. In pursuing work of this 

 kind it soon becomes apparent that the relationships of 

 any form can only be determined with certainty when 

 the conditions that prevail throughout the genus have 

 been examined carefully, and, conversely, that erroneous 



