30 THE MIGRATION OF BIRDS. 



very earliest infancy. The Migration of Birds — 

 a result of Change, as I hope soon to demonstrate 

 — therefore dates most probably from Miocene or 

 even from Eocene ages ; for we know that during 

 those periods vast alterations took place in the 

 relative level of land and sea, that volcanic agency 

 was remarkably active in altering the physical 

 aspect of the northern hemisphere, and that climatic 

 changes, due to varying phases of the earth's eccen- 

 tricity and divergence of ocean currents, were con- 

 siderable. The phenomenon of migration, however, 

 must then have been very different from what it is 

 in our day, and therefore, to understand it in its 

 present aspect, we must pass on to a later period 

 in the earth's eventful history. That in those 

 mighty changes the habit of migration had its 

 origin can scarcely be disputed, but much modifi- 

 cation necessarily took place during the occurrence 

 of the Glacial Epoch succeeding them. This we 

 may call the Post-Pliocene Glacial Epoch. 



That parts of the northern hemisphere have been 

 subjected to a long and severe period of glaciation 

 within comparatively recent geological time, is 

 proved by the fact (among others) that the various 

 species of Mollusca then living are the same as 

 those we meet with in our own age ; not only so, 

 but numerous traces of ice movement have been 

 preserved to us. It is 200,000 years ago since 

 this Glacial Epoch is computed to have reached 

 its maximum, and about 80,000 years since it came 

 to a termination. We need not enter here into 



