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Show 2 3 8 DR. H. GADOW ON MEXICAN [ J u n e 6 , hot-lands and the eastern slopes of the States of Yera Cruz and Chiapas are very wet, with a very long and abundant rainy season, interrupted by a short dry time in the winter. The Pacific side is much drier ; 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 Yolcan de Orizaba, Popocatepetl, Nevado de Toluca, and Nevado de Colima. 2. Immigration and Spreading. 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 plants and animals. We may well ask, what remained of the country as suitable for life. Of course, probably, there were archaic tracts standing out, not affected by these revolutions, but these gneisses, schists, and granites form scattered enclaves. I think it was the Pacific strip-Sonora, Sinaloa, 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 eruptions, but the land was still narrowed ; there was no Atlantic lowland, this being during the whole Miocene epoch, and even later, still below the sea. Consequently we have as available land the western strip as the least altered remnant of Old Sonoraland, and the present eastern limestone belt, beginning with a broad basis in Texas, and extending through Coahuila and Nuevo Leon southwards, narrowing down towards Oaxaca. These were the two belts of land available for spreading southwards. Obviously the Pacific belt is the older of the two, the north-east of Mexico, with Texas, being late Cretaceous terrain. Once arrived in the south of the plateau, there was the essentially granitic, gneissic, and older Cretaceous terrain of Guerrero and Oaxaca, not so much overlaid by volcanic masses. Thence the Great Antillia. afforded easy access into the present Antilles. But it was a long way round from the North. The spreading from South America into this same Antillia was easier in this respect. Later immigrants from the North into Mexico are those of the |
