14 ELEVENTH REPORT. 



less dwarfed representative of another group, the difference in relative size 

 being due to differences in the number of forms between the one in question 

 and the center of origin. 



7. The variations in the characters of each form fluctuate about a mean, 

 and the transition in characters between the different forms is brought about 

 by a gradual variation of the average type. 



Students of evolution problems Tvill 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 generalty associated \vith different environmental conditions. 

 In some cases it seems that these forms may occupy different habitats in 

 the same environment, l^ut in by far the greater number of instances, at 

 least among terrestrial animals, directly related forms inhabit different but 

 neighboring, geographic 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 regions 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 se. 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 be- 

 tween the amount of dwarfing and particular habitats, but that forms (be- 

 longing to different groups) of quite diverse scutellation may occupy 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 become dwarfed), 

 but only act to modify to some extent the invading form, the relative size 

 of the latter being determined as much by the modifications which the grouji 

 has previously undergone as by those to which the particular form has been 

 subjected. 



I believe that these objections to the operation of selection in the evolu- 

 tion of these forms also argue against DeVries's mutation theory as an ex- 

 planation, for, while it is easily conceived that mutations may have arisen 

 within the limits of fluctuating variation in each case, we must also assume 

 that the new form pushed into the new environment, or at least now occupies 

 it to the exclusion of immediately related forms, jjecause better fitted for 

 it, which does not seem to be the case. What seems to have actually taken 

 place is that as each group pushed out from the center of origin it became 

 modified each time it encountered a new region of environmental conditions, 

 not by the selection of forms better adapted to the new conditions, but by 

 the modification of the entire section that invaded the new region. This 

 may be tested more thoroughly by an examination of the method of evolution. 



Perhaps the most striking characteristic of this genus is the manner, in 

 which evolution has taken place along definite lines. Although the forms 

 frecpently have other distinctive characters, they nearly all have this in 

 common that they are more dwarfed than the form from which they have 

 been derived, and there is no case in the genus where a form is larger than 

 its neighbor toward the center of origin. The history of each group has thus 



