156 ESSAYS. 
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.} 
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 oceu- 
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 
1 «Mem. Amer. Acad.,” vol. vi. pp. 377-458 (1859). 
