ii] FEATURES OF ENVIRONMENT 55 



production of new forms of life. Let us apply this 

 principle to geological changes. 



Elevation of a whole country, or of a range of 

 mountains, or a cold epoch following upon a warm 

 one, would be very productive of new forms. The 

 same effect would be produced by the spreading of 

 a glacial epoch from the pole. The tropical creatures, 

 coming under the cooling influence, would change 

 readily, and the old species within the polar circle 

 would be changed into arctic forms, as the glacial 

 wave passed over them. But those which, for some 

 reason or other, were driven south, would remain 

 unchanged because they counteract the new climatic 

 influence by their migration. Such a glacial epoch 

 would thus bring about a great faunistic interming- 

 ling, but it also 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 spell of warm climate spreads again towards 

 the pole, as has taken place during the so-called 

 interglacial epochs? No changes whatever, except 

 that the arctics will die out or remain occasionally 

 . as derelicts, while all the rest, both northerners and 

 southerners alike, will surely reclaim the old ground 

 so far as it suits them. There will be comparatively 

 little making of new species, provided our principle is 

 right, that increase of temperature has a minor effect. 



