MICHIGAN ACADEMY OF SCIENCE. 13 



A CONTRIBUTION TO THE THEORY OF ORTHOGENESIS.^ 



Dr. Alexander G. Ruth yen. 



SeYeral reasons haYe been given why biological discussion has, for a num- 

 ber 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 common knowledge that Darwin 

 considered the natural selection 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 he 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 onh' one. Among other methods- 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 Thamnophis (the garter-snakes).^ 



I will briefly summarize the conditions that prevail in this group: 



1. The genus Thamnophis consists of four groups of closely related forms, 

 each group ranging both northward and southward from the Mexican plateau 

 (which is their center of dispersal) into North and Middle America. 



2. Each group consists of a series of forms, the ranges of which adjoin 

 and correspond to different -geographical regions. 



3. The forms of the same grouj) may intergrade or not, but in either case 

 they come in contact where the geographic conditions with which they are 

 associated meet, and the areas of transition in characters correspond to the 

 areas of intermediate environmental conditions. When the forms intergrade 

 the transition in characters takes place gradually in the intermediate region, 

 and where there is apparently no intergrading at present the two forms 

 become most like each other in this region, there being no abrupt changes 

 in characters l^etween two directly related forms. 



4. Each group tends to l^ecome progressively more dwarfed awa}' from 

 the Mexican plateau, each form usually being more dwarfed than its neighbor 

 toward the center of origin, and less so than the representative whose range 

 adjoins it on the side awa}' from the center. 



5. The relative size is correlated with the number of labials and rows of 

 body scales, and these two characters — size and scutellation — constitute the 

 only ajiparent specific differences, except in the few cases where they are ac- 

 companied l)y differences in color or relative tail-length. 



6. The amount of dwarfing is not associated with particular geographic 

 regions, but the scutellation and relative size of any form is that of its im- 

 mediate ancestor plus the dwarfing which it has itself undergone. Thus 

 a dwarf form of one group frequently occurs in the same region with a much 



^Reprinted from the American Naturalist, XLIII, No. 511. 



-I mention only these three (selection, mutation and orthogenesis) as they appear to me to be tlie 

 only ones that can be considered to have been dominant methods in the evolution of the forms in the 

 genus tliat constitutes the basis of this discussion. 



sRuthven, Alexander G., "Variations and Genetic Relationships of the Garter-snakes," Bull. 61, 

 U. 8. Nat. Museum. 



