824 The American Naturalist. [September, 
conditions. His later papers? extend and confirm the observations 
recorded in these; and the fact that the accumulations and 
research of nearly thirty years afterwards did not change his 
views in any essential particular is of importance. The history 
of the big trees of California, of the forests of the Northern Con- 
tinents, and the peculiarities and resemblance of the North 
American flora as compared with those of Europe and Asia, still 
were shown to point unmistakably to migrations from a former 
common, though extended, area, with subsequent modifications in 
accordance with the theory of descent. 
Dr. Hooker covered a different ground in his study of geo- 
graphical distribution. Taking up successively the Antarctic 
flora, and those of New Zealand, Tasmania, and the Oceanic 
Islands, it was only at a later period in his investigations of the 
floras of Southern Asia and of the Arctic regions that he over- 
lapped in any way the ground already occupied by Dr. Gray. 
His position in regard to theories then prominent was dis- 
tinctly indicated in the * Introductory Essay to the Flora of Tas- 
mania.” ® Referring to the flora of New Zealand," in which he 
had given (though without distinctly endorsing) the prevalent 
view, that species are created as such, he says: “ In the present 
essay I shall advance the opposite hypothesis, that species are 
derivative and unstable." 
Of the observed facts recorded in this series of monographs 
only a few of the most important can be mentioned. 
It was shown in the Flora Antarctica that a certain relationship 
exists between floras of the Antarctic Islands and that of the ex- 
? Three papers of Professor Gray contain his latest contributions to this ee and 
represent his mature views and final a regarding the distribution of plants 1 
Northern Hemisphere. These a 
1. Sequoia and its History: The Relations of North American to Northeast Asi 
to Tertiary Vegetation. A presidential address to the American Association at Dubuque, 
— tdm 
an and 
Geography and Archeology. A lecture delivered before the Harvard Uni- 
inis joe History Society, April, 1 
3. Characteristics of the North American Flora. An address to the botanists of the 
"a Association at Montreal, A 2 
10 Am. Jour. Sci., 1860, Vol. XXIX. 
?! Reviewed in Am. Jour. Sci., 1854, Vol. XVII. 
