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Show 234 DR. H. GADOW ON MEXICAN [June 6, G en e r a l C onclusions. 1. Evolution of Middle America. We have seen in the review of the Amphibian 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. I 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 Palaeozoic 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 Jose 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 /Sonor aland, 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 * S u e s s .-Das Antlitz der Erde. D e L a p p a r e n t .-Traite do Geologie. R. T. H i l l .-" The Geology and Physical Geography of Jamaica : Study of a type 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. Jouru. Sci. vol. xlviii. (1894). J. W. S p e n c e r .-" Reconstruction of the Antillean Continent." Bull. Geol. Soc. America, vol. vi. 1895 ; and Geolog. Mag. 1894, pp. 448-451. A . A g a s s i z .-Reports of the Results of Dredging ...... by the ‘ Blake.' Mem. Mus. Comp. Zool. x. (1883) no. 1, p. 79. J. W. G r e g o r y .--" Contributions to the Palaeontology and Physical Geography of the West Indies." Quart. Journ. Geol. Soc. vol. Ii. (1895) pp. 255-312. J. G. A g u i l e r a .-" Bosquejo Geol6gico de Mexico." Institute Geol6g. de Mexico, pt. 4 (1895) pp. 1-270, with maps. C. S a p p e r .- " Sobre la Geografia fisicayla geologxa de la peninsula de Yucatan." Inst. Geol. Mexico, pt. 3 (1896). |