32 THE MIGRATION 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- 
