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THE AMERICAN NATURALIST [Vol. XLIII 



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 group may intergrade or not, 

 but in either case they come in contact where the geo- 

 graphic 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 between two 

 directly related forms. 



4. Each group tends to become progressively more 

 dwarfed away 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 away 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 apparent 

 specific differences, except in the few cases where they 

 are accompanied by differences in color or relative tail- 

 length. 



6. The amount of dwarfing is not associated with par- 

 ticular geographic regions, but the scutellation and rela- 

 tive size of any form is that of its immediate 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 less dwarfed representative of an- 

 other 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 fluctu- 



