72 Wild Birds and their Haunts 



Aside from these differences of temperature resulting 

 from elevation (" land masses were thrust up above the 

 then existing snow line, sucti 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 tem- 

 perature 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 

 zonal distribution of climates. Birds and beasts travel 

 thousands of hundreds of miles at the oncoming of spring 

 or fall. 



During all the latter part of the long tertiary period the 

 process of mountain building or subsidence was changing 

 the face of Nature. The fauna which had previously 

 enjoyed so wide a range of equable temperature and a 

 general food supply discovered that those ranges had been 

 restricted in certain directions, while perhaps they had 

 become more extended in others. The land bridges which 

 had previously given them passage had sunk beneath the 

 encroaching waters. Mountain ranges had reared their 

 heads into the clouds and were snow-covered, forming 

 impassable barriers in the paths of certain species. The 

 true migration began to take form, keeping pace with the 

 constantly changing climatic conditions, and in general 

 terms they may be said to have begun with the ice age. 



The direct burning rays of the sun near the Equator were 



