SOME GENERAL OBSERVATIONS ON THE WHOLE QUATERNARY. 595 



change of climate, one great cause of change of species has been migra- 

 tion : migration north and south, enforced by change of temperature ; 

 migration in any direction, permitted by change of physical geography. 

 This point is so important, that we must explain it somewhat fully. 



It will be remembered (p. 507) that in Miocene times Greenland, 

 Iceland, and Spitzbergen, were covered with a luxuriant temperate 

 vegetation. The congeners of their vegetation at that time are found 

 now in California, along the shores of the Southern Atlantic States, and 

 in Southern Europe. Evidently at that time there was no polar ice- 

 cap, and therefore no arctic plants. At the end of the Pliocene, the 

 vegetation shows a climate not greatly differing from the present. It 

 is probable, therefore, that the cold had increased until an ice-cap had 

 formed, such as now exists in polar regions, with its accompaniment 

 of arctic species. As the Glacial epoch came on and culminated, the 

 polar ice slowly extended — its margin crept slowly southward, until 

 it reached 40° in America and 50° in Europe, with local extensions 

 stretching still farther southward, in mountain regions. The southern 

 polar regions were probably similarly affected, either simultaneously or 

 alternately. 



We must not confound this movement southward of the southern 

 limit of the ice with the current motion of the ice-sheet itself. The 

 limit of the ice-cap is like the lower limit of a glacier (p. 48). It may 

 be stationary, or advancing or retreating, but the glacial stream flows 

 ever onward. Again, the motion of a glacial current is slow — perhaps 

 one to three feet per day — but the extension or recession of the glacial 

 limit is far slower, perhaps a few feet per annum. We may thus easily 

 appreciate the immense time necessary to advance this limit of the ice- 

 cap to 40° latitude. 



At the end of the Glacial and the commencement of the Champlain 

 epoch a movement of the ice-limit in a contrary direction — a retreat 

 northward — commenced and continued, with perhaps some alternate 

 progressions and regressions, to its present position. 



Now, the effect of this advance and retreat of polar ice upon plants 

 and animals must have been very marked. Temperate plants, inhabit- 

 ing Greenland in the Miocene, were pushed to the shores of the Gulf. 

 Arctic plants — i. e., those which haunt the margin of perpetual ice — 

 were pushed to Middle United States and to Middle Europe ; and arc- 

 tic shells were similarly driven southward, slowly, generation after gen- 

 eration. We say sloivly, for otherwise they must have been destroyed. 

 With the return of temperate conditions, and the retreat of the ice- 

 cap, these species, both shells and plants, again went northward to their 

 appropriate place. But the plant species, and some land invertebrate 

 species, such as insects, had an alternative which the shells had not, 

 viz., to seek arctic conditions also upward on mountains. Many did 



