The ice age in North America and its bearing upon the antiquity of man5th edwith many new maps and illus., enland rewritten to incorporate the facts that bring it up to date, with chapters on Lake Agassiz and the Probable cause of glaciation . e, where no such land-con-nection exists; and, as we shall presently see, tlie problempresented by the distribution of plants in the northern hemi-sphere is very complex and curious. For its solution we arelargely indebted to the sagacitj of the late Professor AsaGray, who discovered the key in tlie influence of the Glacialperiod. In 1857, after he was a
The ice age in North America and its bearing upon the antiquity of man5th edwith many new maps and illus., enland rewritten to incorporate the facts that bring it up to date, with chapters on Lake Agassiz and the Probable cause of glaciation . e, where no such land-con-nection exists; and, as we shall presently see, tlie problempresented by the distribution of plants in the northern hemi-sphere is very complex and curious. For its solution we arelargely indebted to the sagacitj of the late Professor AsaGray, who discovered the key in tlie influence of the Glacialperiod. In 1857, after he was already familiar, from private cor-respondence, with Darwins theory of the origin of species,Professor Gray was called upon to study the extensive botan-ical collections brought back fi-om Japan by the expeditionsof (Joramodores Perry and liodgers. Comparison of thesespecies with those in corresponding latitudes in other por-tions of the world brought out clearly—what had been FLIGHT OF PLANTS AND ANIMALS. 423 before but dimly perceived—that there was a remarkableresemblance between the existing plants of eastern Asia andthose of eastera North America, that more species are com-mon to Japan and Europe than to Japan and western North. o o £2 o Si a ??o XI CO a. I fa America, and that the resemblance is greatest of all betweenJapan and eastern North America. Out of three hundredspecies, common to the temperate regions of eastern i^ pia 424 THE ICE AGE IX yOKTH AMERICA. and the corresponding region of North America, only onethird is represented in western North America.*? The key apphed by Professor Gray for the sohitior. ofthis problem was suggested by the investigations of Heer andothers, which had brought out the fact that, during the Ter-tiary period, just before the beginning of the Ice age, a tem-perate climate, corresponding to that of latitude 35° on theAtlantic coast, extended far up toward the north pole, per-mitting Greenland and Spitzbergen to be covered with tree
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