64 Prof. Asa Gray on Sef^iioia and its History. 



These singular relations attracted my curiosity early in the 

 course of my botanical studies, when comparatively few of 

 them were known, and my serious attention in later years, 

 when I had numerous and new Japanese plants to study in 

 the collections made by Messrs. Williams and Morrow dur- 

 ing Commodore Perry's visit in 1853, and especially by Mr. 

 Charles Wright in Commodore Rodgers's expedition in 1855. 

 I then discussed this subject somewhat fully, and tabulated 

 the facts within my reach*. 



This was before Heer had developed the rich fossil botany 

 of the arctic zone, before the immense antiquity of existing 

 species of plants was recognized, and before the publication 

 of Darwin s now famous volume on the Origin of Species 

 had introduced and familiarized the scientific world with those 

 now current ideas respecting the history and vicissitudes of 

 species, with which I attempted to deal in a tentative and 

 feeble way. 



My speculation was based upon the former glaciation of the 

 northern temperate zone, and the inference of a warmer period 

 preceding (and perhaps following). I considered that our own 

 jn'esent vegetation, or its proximate ancestry, must have occu- 

 pied the arctic and subarctic regions in pliocene times, and 

 that it had been gradually pushed southward as the tempera- 

 ture lowered and the glaciation advanced even beyond its 

 present habitation — that plants of the same stock and kindred, 

 probably ranging round the arctic zone as the present arctic 

 species do, made their forced migration southward upon widely 

 different longitudes, and receded more or less as the climate 

 grew warmer — that the general difference of climate which 

 marks the eastern and the western sides of the continents 

 (the one extreme, the other mean) was doubtless even then 

 established, so that the same species and the same sorts of 

 species would be likely to secure and retain foothold in the 

 similar climates of Jaj^an and the Atlantic United States, but 

 not in intermediate regions of different distribution of heat and 

 moisture, so that different species of the same genus, as in 

 Torreya^ or different genera of the same group, as llcdwood, 

 Taxodium, and GlyjHostrohuSj or different associations of forest 

 trees, might establish themselves each in the region best suited 

 to its particular requirements, while they would fail to do 

 so in any other. These views implied that the sources of our 

 actual vegetation, and the explanation of these peculiarities, 

 were to be sought in and presupposed an ancestry in pliocene 

 or still earlier times occuY)ying the high northern regions. 

 And it was thought that the occurrence of peculiarly North- 

 * Mem. Amer. Acad. vol. vi. 



