32 THE MIGRA TION OF BIRDS. 



servers, often under the direction of committees of learned 

 societies specially appointed for the purpose. The literature 

 of the subject has rapidly increased, and each year sees the 

 publication of elaborate reports giving in detail the move- 

 ments of birds as observed in various countries, especially 

 in Germany, Austria-Hungary, Italy, the British Islands, 

 and the United States and Canada. Much new light has 

 thus been thrown on the cause and the manner of migra- 

 tion, till now the general facts of the subject may be said to 

 be well known. 



The migration of birds evidently dates back to the close 

 of the Tertiary, when great changes in the climatic condi- 

 tions of the northern hemisphere began to prepare the way 

 for the subsequent ice period which buried so large a part 

 of the northern lands under a heavy ice-cap and reduced 

 the present warm temperate latitudes to semi-arctic condi- 

 tions. Birds, in common with other forms of life, were either 

 forced to migrate or suffer extinction under the new condi- 

 tion. As previously a warm temperate or subtropical 

 climate extended northward to Spitzbergen and Greenland, 

 there was no occasion for birds to migrate, and subtropical 

 birds, as well as subtropical plants, found a congenial home 

 almost within the Arctic Circle. Later on the ice-cap 

 melted ; the area of habitable land increased ; but the 

 climatic conditions of the temperate latitudes had become 

 transformed. Instead of a nearly uniform temperature 

 throughout the year, a comparatively warm summer was 

 followed by an icy winter; while a considerable area became 

 opened up as a congenial summer home to a great multitude 

 of birds, the severity of the winter climate forced them to 

 retire to more southern haunts to pass the colder season. 



We have here what seems a natural and reasonable ex- 

 planation of the origin of migration, and as such it is now 

 currently accepted by ornithologists. In this way, it is be- 

 lieved, the habit of migration not only originated but has 

 become so firmly established as to be an irresistible heredi- 



