MICHIGAN ACADEMY OF SCIENCE. 17 



In conclusion I would like to j^oint out that the purpose of this paper 

 is to descriV)e the method of evolution in the genus Thamnophis rather than 

 to discuss the cause of the variations. It is evident that the suggested rela- 

 tion between the action of the environment and the nature of the response of 

 the organism can only be tested by experimentation. 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 development 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 care- 

 fully, and, conversely, that erroneous ideas of relationships are very liable 

 to result from incomplete knowledge of the course of evolution in the group, 

 so that it is very hazardous to select a few forms of a genus and endeavor to 

 discover the laws governing their development. An illustration can be drawn 

 from this genus: Butler i, which lies entirely within the range of sirtcdis, 

 may, so far as its characters go, easily be conceived to have l^een derived 

 from the latter, and, if so, a different theory of the factors involved in its evo- 

 lution must he sought to exjjlain its origin. That it has more probably 

 been developed from radix by dwarfing only becomes relatively certain 

 Avhen the lines of evolution in the genus have been worked out. This 

 should discourage attem])ts to adduce as evidence for or against any theory 

 the relationships of particular forms before the genus has been studied as a 

 whole, and the general lines of development determined. 



