710 Hans GADow, 
by ascent into the cold zone than by descent into the 
hot lands. This agrees with the chief result drawn from the 
distribution of the Southerners. 
If the main conelusion that change into colder 
environment, or ascent, is a more powerful factor 
than descent, allows of general application it would 
be of far reaching importance. To make sure of this it 
will have to to tested carefully and without bias by scrutiny of 
the faunas of various countries. There are no doubt many exceptions 
which may restriet the conclusion that “temperate” genera produce 
less easily species which are suited exclusively for hot-country life 
than species for colder regions. For instance of the two dozen 
species of Spelerpes about 6 seem to be restrieted to cold zones, 
whilst there are perhaps 4 which apparently are found in hot 
countries only. Of the 21 American species of Tropidonotus about 
5 seem to be restrieted to a warm or hot climate, and a census of 
the total number of species of this genus, about 70, shows that about 
half are tropical, many of them being exclusively dwellers in hot 
countries. Therefore tbis genus might easily be taken to upset the 
above conclusion. But it is a genus with an almost cosmopolitan 
range, and it would be rash to say where it has arisen. It may 
have originated under typically tropical conditions. However, when 
we restrict our scrutiny to America, the quest assumes a reasonable 
aspect. No Tropidonotus whatever exists in South America, whilst 
many of them occur in North America, and very few in Central 
America. Consequently it is reasonable to assume that so far as. 
Mexico is concerned, these snakes are Northerners which have ex- 
tended their range southwards. 
Why should change from a hot into a cool climate be more 
easily effected, without harm to the experimenting individual, than 
the reverse? And why should ascent be more productive of specific 
change than descent? These two ideas seem to be contradictory. 
It is well known that most creatures can endure a temporary 
change into cooler surroundings, although they may not flourish, 
while the reverse of such conditions prostrates and often kills them. 
Amphibia are especially sensitive in this respect; they may be 
almost frozen, becoming quite lethargie, and they will revive, but 
a few extra degrees of heat may quickly kill them. Many tropical 
plants can be cultivated in temperate countries where they have 
to adapt their oeconomy to shorter summers, whilst Northern plants, 
