234 DR. H. GADOW ON MEXICAN [June 6, 
GENERAL CONCLUSIONS. 
1. Hvolution of Middle America. 
We have seen in the review of the Aimphibian and Reptilian 
fauna of Mexico that it is composed of Northern and Southern 
immigrants; that a considerable number of the northern group 
can claim to be old, autochthonous Nearctics ; that some families, 
genera, or species have also representatives in the Antilles, and 
that most of these forms point unmistakably to Central America, 
or even further south, as their original home; lastly, that but few 
Antilleans belong to a northern stock. 
The explanation lies in the geological history of this part of the 
world. J restrict myself on purpose to this part, lest such an 
inquiry should lead to a discussion of the whole globe since the 
first dawn of Amphibian life in some Paleozoic country. 
Our present task limits itself to the Tertiary period. It is 
doubtful whether any of the genera in question are older than the 
Eocene, but not a few can be proved to have existed in our region 
in the mid-Miocene epoch; and it is surprising that they should 
date so far back. Lastly, there was no Central America in the 
Cretaceous period. 
The building up of Mexico and neighbouring countries seems 
to have taken place as follows, so far as I can gather from the 
writings of A. Agassiz, Suess, Lapparent, R. T. Hill, J. W. 
Spencer, J. W. Gregory, C. Sapper, and José G. Aguilera *. 
The accompanying consecutive series of maps illustrate my 
abstract conclusions, and only in this abstracted sense can claim 
originality. 
Mexico came into existence during the Lower Cretaceous epoch. 
To a nucleus of land, Sierra Nevada and California, were added 
the Rocky Mountains and the bulk of the Mexican Plateau. 
This large complex I call the Old Sonoraland, It is important to 
remember that it was separated, during the Upper Cretaceous 
epoch, by a broad belt of sea from the eastern and northern parts 
of North America. A third mass of land existed as Brazilialand. 
In the meantime appeared Antillean lands, and, possibly in 
sympathy with the east to west trending mountains of Honduras 
* SuEss.—Das Antlitz der Erde. Dr LapparEent.—Traité de Géologie. 
R. T. Hint.—*‘ The Geology and Physical Geography of Jamaica: Study of atype 
of Antillean development.” Bull. Mus. Comp. Zool. Harvard, xxiv. (1899) pp. 1-226. 
See also other papers in same Bulletin, xvi. (1895), and in Amer. Journ. Sci. 
vol. xlviii. (1894). 
J. W. Spencer.— Reconstruction of the Antillean Continent.” Bull. Geol. Soc. 
America, vol. vi. 1895 ; and Geolog. Mag. 1894, pp. 448-451. 
A. Agassiz.—Reports of the Results of Dredging ...... by the‘ Blake. Mem. 
Mus. Comp. Zool. x. (1883) no. 1, p. 79. 
J. W. Grecory.— Contributions to the Paleontology and Physical Geography 
of the West Indies.” Quart. Journ. Geol. Soc. vol. li. (1895) pp. 255-312. 
J. G. AGUILERA.—* Bosquejo Geolégico de Mexico.” Instituto Geolés. de Mexico, 
pt. 4 (1895) pp. 1-270, with maps. 
C. SAPPER.—“ Sobre la Geografia fisica y la geologia de la peninsulade Yucatan.” 
Inst. Geol. Mexico, pt. 3 (1896). 
