No. 511] THE THEORY OF ORTHOGENESIS 



in:; 



ate about a mean, and the transition in characters between 

 the different forms is brought about by a gradual varia- 

 tion of the average type. 



Students of evolution problems will recognize in this 

 summary (a) the old story of the association of different 

 forms with different environmental conditions, (b) the 

 so-called definite or determinate evolution, and (c) that 

 the phylogenetic variations are gradual. 



It has long been noted that, among animals of the same 

 group, the different forms are generally associated with 

 different environmental conditions. In some cases it 

 seems that these forms may occupy different habitats in 

 the same environment, but in by far the greater number 

 of instances, at least among terrestrial animals, directly 

 related forms inhabit different, but neighboring, geo- 

 graphic areas. That there is some connection between the 

 differentiation of such a group and the diversities of the 

 region it occupies has usually been assumed; at least it 

 is a fact that must be considered in any explanation of 

 evolution. It is explained by natural selection on the 

 assumption that the different conditions in the two re- 

 gions demand different adaptations on the part of the 

 organism, but this explanation will apparently not hold 

 in the case of the garter-snakes, for there is certainly no 

 advantage in dwarfing per sc. It might be assumed (and 

 it would be pure assumption) that this trait is correlated 

 with unperceived adaptative characters, but this would 

 seemingly be trying harder to save the theory of natural 

 selection than to explain the facts. I have pointed out 

 that there is no relation between the amount of dwarfing 

 and particular habitats, but that forms (belonging to 

 different groups) of quite diverse scutellation may oc- 

 cupy the same region. Apparently the conditions in 

 each region do not call for a particular size (as would, 

 it seems to me, be approximately the case if the struggle 

 for existence in each habitat required that the forms be- 

 come dwarfed), but only act to modify to some extent 

 the invading form, the relative size of the latter being 



