238 DR. H. GADOW ON MEXICAN [Jmie 6, 



hot-lands and the eastern slopes of the States of Vera Cruz and 

 Chiapas are very wet, with a very long and abvindant rainy season, 

 intei-rupted by a short dry time in the winter. The Paciiic side is 

 much di'ier ; the actual amount of annual rainfall is considerably 

 less and the dry winter period is much longer. 



The plateau rises from less than 1000 feet near Laredo, and 

 3800 at El Paso, gradually to about 6000 at Aguas Calientes and 

 Queretaro, and above 7000 at Mexico City and Puebla. The 

 highest masses of mountains, bordering the plateau, lie in the 

 south-east, south and west, culminating in the snow-capped peaks 

 of Citlaltepetl or Volcan de Orizaba, Popocatepetl, Nevado de 

 Toluca, and Nevado de Colima. 



2. Immigration and Sjweading. 



Obviously these physical conditions influence the fauna now ; 

 what they were like in bygone ages we can only surmise. Ranges 

 of mountains are by no means always barriers ; on the contrary, 

 they help the dispersal along the lines of their long axes. Regions 

 covered by the sea are of course not available. The same applies 

 to districts which are subject to volcanic eruptions. This is very 

 important for Mexico. Not only the Western Sierra Madre with 

 its continuations to Colima and thence towards Puebla, but also 

 almost the whole of the plateau became covered with eruptive 

 masses, and, considering the immense extent of this terrain, a long- 

 time must have elapsed before it became available for j)]ants and 

 animals. We may well ask, what remained of the country as suit- 

 able for life. Of course, pro1)ably, there were archaic tracts 

 standing out, not affected by these revolutions, but these gneisses, 

 schists, and gi'anites form scattei-ed enclaves. I think it was the 

 Pacific strip — tSonora, tSinaloa, Tepic, and part of Jalisco — which 

 was not affected ; in fact, the Pacific slopes, together with the land 

 which has since sunk below the Gulf of California. On the eastern 

 side, part of the plateau did not suffer from ei'uptions, but the 

 land was still narrowed ; there was no Atlantic lowland, this 

 being during the whole Miocene ejioch, and eA'en later, still below 

 the sea. Consequently Ave have as available land the western strip 

 as the least altered I'emnant of Old Bonoraland, and the present 

 eastern limestone belt, beginning with a broad basis in Texas, and 

 extending through Coahuila and Nuevo Leon southwards, narrow- 

 ing down towards Oaxaca. These Avere the two belts of land 

 available for spreading soutliAvards. Obviously the Pacific belt is 

 the older of the tAvo, the north-east of Mexico, Avitli Texas, being 

 late Cretaceous terrain. Once arriA^ed in the south of the plateau, 

 there Avas the essentially granitic, gneissic, and older Cretaceous 

 tei^rain of Gueri'ero and Oaxaca, not so much overlaid by volcanic 

 masses. Thence the Great Antillia afforded easy access into the 

 present Antilles. But it Avas a long Avay round from the North, 

 The spreading from South America into this same Antillia Avas 

 easier in this respect. 



Later immigrants from the ISToi'th into Mexico are those of the 



