708 Hans GADow, 
equal to a difference in altitude of 4500 feet. Caeteris paribus, 
and in theory, an elevation of 9000 feet in Southern Mexico has the 
same mean annual temperature of a lowland country 20° further 
North, let us say of Missouri. 
The division into Southerners and Northerners would be fair 
enough if the latter were composed of Nearctics only but the 
Sonorans are the disturbing element. As pointed out before, 
they are in some respect intermediate, and above all our know- 
ledge is not sufficient to discern in every case between them 
and Nearctics, although a look at the appended lists shows that 
these two groups represent two different elements. The old Sonora- 
land probably continued northwards through California and possibly 
thence, around the Pacific, was continuous with Eastern Asia. In 
any case it was and is part of the Pacific division of the 
Northern hemisphere and it is most likely that the Ambly- 
stomatinae were some of its oldest inhabitants. That is a question 
of geological time. On the other hand the Eastern and Northern 
half of North America is a part of the Atlantic world, more 
akin to Europe and West Siberia. But since North America 
became one consolidated entity, the Pacific and Atlantic, or if it be 
preferred, the Pacific or Western and the Atlantic or Eastern faunas 
have become mixed, perhaps inextricably. 
The „Nearctic‘“ species. 
(List C). 
There are 24 Nearctic forms, i. e. species the genera of which, 
we have reason to presume, came into the country from the North, 
but not from Sonoraland. Of these Nearctics 6 are modified for the 
cold zone, 5 of them being restricted to levels above 7000 ft. 8 Species 
extend down to sealevel, two of which (Rana palmipes and Zamenis 
mexicanus) have shifted their centre of altitudinal range to a level 
just within the hot zone, but they have not become exclusively hot 
country species, as they are still found at the 5000’ level. This case 
is instructive. If we did not feel sure that Rana is an originally 
typical Northern genus, we should declare without hesitation, that 
a creature like R. palmipes was autochthonous of the Hotlands whence 
it extends upwards well into the temperate zone. In reality it is 
a member of a Northern genus, but behaves now like a Southern, 
Central American, species. 
The Nearctics comprise 9 short-ranged and 14 long-ranged species. 
