112 Hans GaDow, 
logical changes. Elevation of a whole country, or of a range of 
mountains, or a cold period following upon a warm one would be 
most productive of new forms; and the same effect should be pro- 
duced by the spreading of a socalled glacial epoch from the pole. 
The tropical creatures, coming under the cooling influence will change 
readily, and the polar species, as the wave passes over them, will 
be changed into arctie forms, but those which for some reason or 
other are driven South would remain unchanged, either because they 
counteract the new climatic influence by their migration, or because, 
(even if they should spread into hot countries) they can accommodate 
themselves to their climate, aceyrding to the principle mentioned above. 
Such a glacial epoch would thus bring about not only a great 
faunistic intermingling but would actually produce new forms, namely 
arctics and those transformed Southerners which did not, or could 
not, withdraw. 
What would happen with the turn of the tide, when a warm 
climate spreads again towards the pole? No changes whatever, except 
that the arctics will die out or remain occasionally as derelicts, 
while all the rest, both Northerners and Tropicals alike, will surely 
reclaim the old ground so far as it suits them; there will be com- ' 
paratively little making of new species, provided our principle is right 
that increase of temperature has a minor effect. 
Such speculations must not be driven to far. It would be silly 
to conclude that cold is a more favourable factor to life than warmth, 
but it is quite a different question whether a change from hot to 
cold may not have a profoundly stirring influence upon organisms, 
a case of either change or die. The Permian epoch was one of 
widely spread coolness and played great havock at least with the 
marine fauna, by reducing its numbers of individuals and species, 
but it also ushered in, or prepared, a new and remarkable terrestrial 
vertebrate fauna. Our last northern glacial epoch may have killed out 
much of the warm miocene life, but it has given us the present Arctic 
fauna, which is very considerable and remarkable for being quite 
up to date, singularly free from oldfashioned types. The place for 
these are the tropies, because there the climatic conditions have 
changed least. There are the Hotlands and they have been hot since 
at least Permian times, and if, for all we know to the contrary, they 
should have been too hot for typical terrestrial life, then the hot and 
life sustaining belt would simply have to be shifted away from the 
equator, to suit our lively imagination. 
