iil INTRODUCTION. 
Mexico, extending along the Central-American plateau to the extreme limits of our 
area, and even beyond. ‘This southward extension of northern types is due partly to 
the identical physical conditions of the arid tableland of Sonora and Chihuahua, 
which is merely a continuation of that of Arizona and New Mexico, and partly to 
the great altitude and temperate climate of the Central-American plateau. 
Thus, a boundary-line between the North- and South-American regions cannot be 
drawn; the whole of Central America is a transition-tract which, unlike any other 
part of the world, shows the most extraordinary diversity of climatic, physical, and 
meteoric conditions within comparatively small areas, favouring the evolution of a great 
variety of types of genera and species, and influencing the dispersal of immigrants from 
the North and South. To divide Central America into zoological provinces delimited 
by the distribution of the genera and species of Reptiles and Batrachians isa task which, 
in the present state of our knowledge, would appear to be rather premature. Cope 
attempted it for a part of Mexico as recently as 1896. Ina paper on the geographical 
distribution of the Batrachia and Reptilia of North America he allotted Central 
and Northern Mexico to his Sonoran, Austroriparian, Toltecan (and Tamaulipan) 
subregions, including in the Sonoran Chihuahua, and “perhaps” Durango and 
Sinaloa, and subdividing the Toltecan into an Austroriental, Austrocentral, and 
Austroccidental district. But this division cannot be accepted with confidence, 
because, as he himself points out, large unexplored districts intervene between 
these various divisions, and because our knowledge of the range of the species, 
and of the individual species themselves, is too fragmentary. 
To show the range of the 695 species enumerated in the present volume, I have 
prepared the subjoined Table (pp. x-xvii) on the plan adopted by Mr. Godman for the 
genera of Rhopalocera, with one or two modifications. This shows in two columns 
whether a species extends beyond the southern or northern limit of the Central- 
American area: Mexico is divided into Northern and Southern Mexico *, Tampico on 
the Atlantic and Mazatlan on the Pacific coast being grouped with Northern Mexico, 
as in Mr. Godman’s table; but I have given to the Yucatan peninsula a separate 
column, on account of its large extent and its markedly different features, the country. 
being uniformly low, very flat, and dry; British Honduras or Belize I have grouped 
* The species recorded from “ Mexico,” without definite locality, are placed under Southern Mexico, 
excepting those of known northern distribution. | 
