234 . DR. H. GADOw ON MEXICAN [June 6, 



General Conclusions. 

 1. Evolution of Middle A^nerica. 



We have seen in the review of the Amphibian and Reptilian 

 fauna of Mexico that it is composed of ISTorthern and Southei'n 

 immigrants ; that a considerable number of the northern group 

 can claim to be old, autochthonous IN earctics ; that some families, 

 genera, or species have also representatives in the Antilles, and 

 that most of these forms point unmistakably to Oenti'al 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 

 fii'st 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, wei'e added 

 the Rocky Mountains and the bulk of the Mexican Plateau. 

 This large complex I call the. Old Sonorcdand. It is imjoortant to 

 I'emember 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 Hondui-as 



* Suess. — Das Antlitz der Erde. De Lappaeent. — Traite de Geologie. 



R. T. Hill. — ■" The Geology and Plij^sical Geograplij^ 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. Journ. Sci. 

 vol. xlviii. (1894). 



J. W. Spencee. — " Reconstruction of the Antillean Continent." Bull. Geol. See. 

 America, vol. vi. 1895 ; and Gcolog. Mag. 1894, pp. 448-451. 



A. A.GASSiz. — Reports of the Results of Dredging by the ' Blake.' Mem. 



Mus. Comp. Zool. x. (1883) no. 1, p. 79. 



J. W. Geegoey. — " Contributions to the Palaeontology and Physical Geography 

 of the West Indies." Quart. Journ. Geol. Soc. vol. li. (1895) pp. 255-313. 



J. G. Aguilbea. — ■" Bosquejo Geologico de Mexico." Instituto Geolog. de Mexico, 

 pt. 4 (1895) pp. 1-270, with maps. 



C. Sappee. — " Sobre la Geograffa fisicay la geologia de la peninsula de Yucatan." 

 Inst. Geol. Mexico, pt. 3 (1896). 



