826 The American Naturalist. [September, 
have numerous and very remarkable species peculiar to them, 
and which distinguish them from the continental islands. He 
discusses at length the possibility of transoceanic communication, 
and although still impressed with the difficulty of accounting for 
the distribution of plants on oceanic islands by reference to 
agencies now in operation, he is far less inclined to deny that 
these may be sufficient than in his discussion of the floras of 
New Zealand and Tasmania. In fact, he seems ready to admit 
the full force of the argument, as recently stated by Wallace, for 
their distribution by natural agencies now acting, although there 
were still certain difficulties that did not seem to him to readily 
yield themselves to such an explanation. 
Hooker's extended and long-continued study of the distribu- 
tion of plants in every part of the Eastern Hemisphere had led him 
to essentially the same conclusions as those reached by Dr. Gray. 
Both had come perforce to think of species as unstable, and both, 
while recognizing to the full extent the action of existing agencies 
of dispersal, had felt the necessity of assuming the action of 
climatic changes antedating the present geological epoch, the 
results of these changes being in a good degree definite and 
ascertainable in the Northern hemisphere, less definite and more 
perplexing in the Southern. 
The most voluminous writer, and the one who has perhaps done 
the most, taken all in all, to advance our knowledge of the dis- 
tribution of plants, was George Bentham, who for fifty-seven 
years, ending with his death in 1883, contrived to produce, one 
after another, floras, monographs, and other botanical papers, 
until even a review of them become a herculean task. 
He approached the subject differently from either Gray or 
Hooker. Finally recognizing, equally with them, the importance of 
the theory of descent as an essential factor, he undertook to apply 
this by a laborious and exhaustive comparison of botanical 
characters and actual geographical location of species, genera, 
and sub-orders. “If,” he says, “the two theories be admitted, 
that allied species and genera have a common origin, and n 
the descendants of a common stock placed in different region? 
having no inter-communication will vary in these different regions 
