156 ESS A YS. 



Whether the Japanese and the Alleghanian plants are 

 exactly the same or not, it needs complete specimens of the 

 two to settle. So far as we know, they are just alike ; and 

 even if some difference were discerned between them, it would 

 not appreciably alter the question as to how such a result 

 came to pass. Each and every one of the analogous cases I 

 have been detailing — and very many more could be men- 

 tioned — raises the same question, and would be satisfied 

 with the same answer. 



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. 1 



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 moderate 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 

 present 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 be} r ond 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 

 1 "Mem. Amer. Acad.," vol. vi. pp. 377-458 (1859). 



