MAMMALS OP THE MEXICAN BOUNDARY. 73 



DIFFERENTIATION TRACTS.a 



When the collection of mammals from the Mexican Boundary was 

 assembled, it became obvious immediately that all of the forms varied 

 geographically and that certain regions stamped certain peculiari- 

 ties upon all of the species inhabiting them. Pallid forms came 

 always from the two deserts, and dark forms from the elevated tract 

 which separates them and from the coastal regions bordering the Gulf 

 of Mexico and the Pacific Ocean. Again, the pallid forms from the 

 high Eastern Desert Tract differed from the pallid races of the same 

 species in the low Western Desert Tract, and the darker forms from 

 the Elevated Central Tract differed appreciably from those of the 

 Middle Texan Tract and the Pacific Coast Tract. Besides these five 

 principal tracts, which cover almost the whole extent of the Mexican 

 Boundary, it was found that the elements of differentiation tracts 

 penetrate the line from north or south at several points, and these 

 have been styled Minor Differentiation Tracts, as they are not fairly 

 bisected by the Mexican Boundary Line. The islands off either ex- 

 tremity of the Boundary Line also furnish peculiar mammal forms, 

 and have been called Insular Differentiation Tracts. As a whole, the 

 mammals' conform regularly to the characteristics impressed upon 

 them by differences in these tracts. It is certain that no mammal is 

 precisely the same in any two of the five principal tracts, although 

 the degree of variation — which is by no means confined to differences 

 in color, but extends to form, proportion, and size — ^varies in differ- 

 ent groups and species. In many cases species pass through a regu- 

 lar intergradation, on the borderland, in passing from one tract to 

 the next. In this work I have characterized these intergrading 

 forms as subspecies, distinguishing as full species overlapping and 

 separated forms, when the latter are not known to intergrade in 

 regions north'*6r south of the line. In the order of occurrence, from 

 east to west, the tracts are as given below : * 



Padre Island Tract. 



Tamaulipan Subtropical Tract. 



Middle Texan Tract. 



Eastern Desert Tract. 



Elevated Central Tract. 



Taqui Basin Subtropical Tract. 



Western Desert Tract. 



Calif ornian Subtropical Tract. 



Pacific Coast Tract. 



Santa Barbara Island Tract. 



a See Plate II. 



»Tlie detailed description of collecting stations (pp. 74 to 142) gives a fair 

 idea of the zoographic features of the several tracts. 



