GLACIAL EPOCHS AND POLAR CLIMATES. 37 



little less than 2,000,000 years ago, and another 

 about 850,000 years ago, which probably correspond 

 in time with the Eocene and Miocene Periods 

 respectively. Owing to the physical aspect of the 

 great masses of continental land during these ages 

 being such as to admit not one but several warm 

 ocean currents into the Polar area, the differences 

 of climate produced by precession and these great 

 changes of eccentricity would not be very marked. 

 As Wallace remarks : " the summers would be at 

 one period almost tropical, at the other of a more 

 mild and uniform temperate character ; while the 

 winters would be at one time somewhat longer and 

 colder, but never probably more severe than they 

 are now in the West of Scotland." Gradually 

 through the Pliocene Period the eccentricity of the 

 earth's orbit again became exceptionally high, and 

 finally culminated in the Post-Pliocene Glacial 

 Epoch which was brought about by this and other 

 causes, already noted. It will thus be seen that 

 although the climate never became glacial during 

 the Tertiary Period, great changes took place, per- 

 haps not sufficiently severe to lead to a very decided 

 Migration of birds, but enough to initiate the habit 

 in a comparatively small degree. 



That the Glacial Epoch had an incomparably 

 deeper and more lasting effect upon the movements 

 of birds than any other climatic change during Avian 

 history, is unquestionable. As this grand phenome- 

 non progressed, rendering the climate of the Polar 

 and temperate zones more and more rigorous, the 



