440 DISTRIBUTION OF PLANTS IN SOUTHERN MEXICO, 
Mexico, although genetically part of the North American continent, has 
received its present fauna from both North and South America. The 
Northern fauna, which includes the Sonoran, and therefore what may be 
called the endemic Mexican stock, is the oldest arrival and its species belong 
now mostly to the cool and cold zone: the fauna of the plateaus is made up 
essentially of those Northerners which have spread, although thinly, over 
the elevated plains; some have remained in the temperate zone and a few 
have passed into the tropics. The Southern fauna has surged northwards 
through the Tierra caliente, both east and west of the plateau. This plateau 
as such presents a most effective barrier to the Southerners, few of them 
having to a small extent spread on to it, whilst some Southern species have 
succeeded in reaching greater elevations on those mountains which slope 
into the tropics. Lastly, those few species—about 15 to 20—which are 
restricted to the temperate zone and yet are not ancient endemies, show that, 
for Reptiles and Amphibia, the temperate belt is rather no-man's land than the 
happy medium favourable to the majority. 
