CHAPTEE XXXVIII 



AMERICA : AND GEOGRAPHICAL DISTRIBUTION 



THE outstanding event of 1877 was the long looked for visit 

 to the United States. This had been half-planned for the 

 last four years. America had a two-fold call for Hooker, in 

 the problem of the North American Flora and the friendship 

 with Asa Gray. These represented the personal and impersonal 

 sides of the same impulse. Their common interest in the same 

 question, approached from different sides, had initiated an 

 unbroken correspondence which deepened in personal and 

 scientific interest with their united appreciation of Darwin. 

 Gray had already visited England four times, and was urgent 

 for Hooker to come over and join him in a personal study of 

 the complexities of botanical distribution in the States. 



Two features in the problem which cried most loudly for 

 explanation were the remarkable connexion between the 

 plants of the Eastern States and those of Eastern Asia and 

 Japan with no living intermediate connexions and the 

 hard line of division between the Arctic floras of America 

 and Greenland. Independently of each other, Gray had in- 

 vestigated the former 1 and Hooker the latter. 2 Both came 

 back to a common cause in the Glacial Period and the earlier 

 land connexion with an Arctic continent. 



But why had not the Glacial Period produced the same 



1 ' Observations upon the Relations of the Japanese Flora to that of 

 North America, and of other parts of the North Temperate Zone,' Memoirs of 

 the North American Academy of Sciences, vol. vi. p. 377. Bead December 14, 

 1858, and January 11, 1859. 



* ' Outlines of the Distribution of Arctic Plants.' Read before the Linn. 

 Soc., June 21, 1860. Trans. Linn. Soc., xxiii. p. 257. 

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