THE BOOK OF MIGRATORY BIRDS 65 



of the same general temperature everywhere from pole to 

 pole. First, that there was an epoch of torrid heat followed 

 by one of tropical heat and succeeded by one of temperate 

 heat, which gradually passed into one of excessive cold, 

 during which period the higher lands were snow-covered. 

 From these regions descended those immense glaciers, 

 many reaching the sea level. Since this ice age climates 

 have become zonal a condition which seems to us most 

 natural, because man remembers naught to the contrary. 



The geological record shows us, however, that every- 

 where from pole to pole the same life existed during all 

 the periods before the latter part of the temperate tertiary 

 epoch. This implies that the ranges of all the various 

 species were then vastly wider. Fossil sequoias and 

 poplars found in the Arctic complete families mostly found 

 on the Pacific coast. 



Aside from these differences of temperature resulting 

 from elevation ("land masses were thrust up above the 

 then existing snow line, such snow line being independent 

 of latitude"), there were, in the nature of things, few 

 reasons for migrations of either fauna or flora. The first 

 wanderings would have been due to these causes, and were 

 undoubtedly as limited as these causes. 



When finally the gradual transition from earth-heat 

 control to sun-heat control had taken place, and the ice 

 age began, these wanderings to and fro become systematic 

 and periodical. The stronger and more active individuals 

 pushed further on than their fellows, or they climbed up 

 further on the mountain sides, thereby forming a class 

 apart. They mated and founded new varieties. This 

 process of natural selection continued for many thousands 

 of years. The fur-bearing animals were in time produced, 

 and as their protective coverings increased in warmth these 

 extended their search for such conditions of temperature 

 or of food supply as their well-being demanded. 



So here we have in its earliest and simplest form the 

 origin of the migratory movements of animals which have 

 developed to such an extent in this day under the present 



