32 THE MIGRATION OF BIRDS. 



glaciation having ever previously occurred in north 

 polar regions within geological time, although this 

 view is not shared by many scientists, including 

 Dr. Croll, who persists in his theory of intermittent 

 or alternate glaciation following phases of high earth 

 eccentricity. As Professor Nordenskjold remarks 

 {Geological Magazine, 1^75) — "An examination of 

 the geognostic condition, and an investigation of 

 the fossil flora and fauna of the polar lands, show 

 no sign of a glacial era having existed in those 

 parts before the termination of the Miocene Period, 

 [therefore] we are fully justified in rejecting, on 

 the evidence of actual observation, the hypothesis 

 founded on purely theoretical speculations, which 

 assume the many times repeated alternation of 

 warm and glacial climates between the present time 

 and the earliest geological ages." 



The Post-Pliocene Glacial Epoch seems to have 

 been primarily brought about as follows. During 

 the latter part of the Secondary Period a consider- 

 able elevation of land occurred in the Arctic regions, 

 apparently the climax of a slow compact develop- 

 ment of continental land, which not only barred 

 several of the water passages to the north, but 

 probably checked the flow of various warm ocean 

 currents to this polar area, simultaneously with or 

 just previous to a long phase of high eccentricity 

 of the earth's orbit. From this evidence we may 

 justly infer that birds have had only one experience 

 of a Glacial Epoch, for the one preceding it was 

 probably, if at all, during the Permian Period in 



