A CONTRIBUTION TO THE THEORY OF 

 ORTHOGENESIS 1 



DR. ALEXANDER G. RUTHVEN 

 University of Michigan Museum 



Several reasons have been given why biological dis- 

 cussion has, for a number of years, ceased to center about 

 the fact of evolution and is now chiefly concerned with the 

 factors, for such is evidently the case: the principal aim of 

 modern biological researches is apparently to throw light 

 upon the question of method. It is now a part of com- 

 mon knowledge that Darwin considered the natural selec- 

 tion of fluctuating variations to be the principal factor 

 in evolution, and some of his successors have gone so 

 far as to see in it a sufficient one ; but, while few biologists 

 will probably be disposed to deny that natural selection 

 is an efficient factor in evolution, there seems to be now 

 on hand a sufficient body of data to show that it is far 

 from being the only one. Among other methods 2 that 

 have been emphasized, mutation and orthogenesis may be 

 mentioned, each of which has its adherents, and it is the 

 last named of these that seems to be the principal one 

 concerned in the evolution of a group of snakes that I 

 have recently monographed — the genus TJuniniopJiis ( the 



I will briefly summarize the conditions that prevail 

 in this group : 



1. The genus Thamnophis consists of four groups of 



1 Read at the Darwinian Celebration of the Michigan Academy of Science, 

 April 1, 1909. 



S I mention only these three (selection, mutation and orthogenesis), as 



