238 DR, H. GADOW ON MEXICAN [June 6, 
hot-lands and the eastern slopes of the States of Vera Cruz and 
Chiapas are very wet, with a very long and abundant rainy season, 
interrupted by ashort dry time inthe 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 
Querétaro, 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 Volean 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 alway s barriers ; on the contrar 
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 voleanic 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 suit- 
able 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 
Pacifie strip—Sonora, Sinaloa, Tepic, and part of Jaliseo—which 
was not affected ; m 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, narrow- 
ing 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 
