CHAPTER XVII. 



FLIGHT OF PLANTS AND ANIMALS DURING THE GLACIAL 



AGE. 



Among the most interesting incidental effects of the Gla- 

 cial period is that of its influence in distributing plants and 

 animals over the lower latitudes. A glance at a polar pro- 

 jection of the northern hemisphere shows to what a remark- 

 able extent the land is clustered around the north pole, and 

 how easy it would have been, under favorable conditions of 

 climate, for plants and animals to spread from that vicinity 

 along different meridians, till, in the lower latitudes, they 

 should be on opposite sides of the earth. In conformity 

 with these natural lines of emigration, it has long been 

 known that both among plants and animals the species of 

 the northern hemisphere are much more closely allied than 

 those of the southern hemisphere, where no such land-con- 

 nection exists ; and, as we shall presently see, the problem 

 presented by the distribution of plants in the northern hemi- 

 sphere is very complex and curious. For its solution we are 

 largely indebted to the sagacity of the late Professor Asa 

 Gray, who discovered the key in the influence of the Glacial 

 period. 



In 1857, after he was already familiar, from private cor- 

 respondence, with Darw T in's theory of the origin of species, 

 Professor Gray was called upon to study the extensive botan- 

 ical collections brought back from Japan by the expeditions 

 of Commodores Perry and Rodgers. Comparison of these 

 species with those in corresponding latitudes in other por- 

 tions of the world brought out clearly — what had been 



