306 Geographical Distribution of Plants 
At last the ground was cleared and we are led to the final 
conclusion. “If the difficulties be not insuperable in admitting that 
in the long course of time all the individuals of the same species 
belonging to the same genus, have proceeded from some one source; 
then all the grand leading facts of geographical distribution are 
explicable on the theory of migration, together with subsequent 
modification and the multiplication of new forms.” In this single 
sentence Darwin has stated a theory which, as his son F. Darwin 
has said with justice, has “revolutionized botanical geography*.” It 
explains how physical barriers separate and form botanical regions; 
how allied species become concentrated in the same areas; how, 
under similar physical conditions, plants may be essentially dissimilar, 
showing that descent and not the surroundings is the controlling 
factor ; how insular floras have acquired their peculiarities; in short 
how the most various and apparently uncorrelated problems fall 
easily and inevitably into line. 
The argument from plant distribution was in fact irresistible. 
A proof, if one were wanted, was the immediate conversion of what 
Hooker called “the stern keen intellect?” of Bentham, by general 
consent the leading botanical systematist at the time. It is a striking 
historical fact that a paper of his own had been set down for reading 
at the Linnean Society on the same day as Darwin’s, but had to 
give way. In this he advocated the fixity of species. He withdrew 
it after hearing Darwin’s. We can hardly realise now the momentous 
effect on the scientific thought of the day of the announcement of the 
new theory. Years afterwards (1882) Bentham, notwithstanding his 
habitual restraint, could not write of it without emotion. “I was 
forced, however reluctantly, to give up my long-cherished convictions, 
the results of much labour and study.” The revelation came without 
preparation. Darwin, he wrote, “never made any communications 
to me in relation to his views and labours.” But, he adds, “TI...fully 
adopted his theories and conclusions, notwithstanding the severe 
pain and disappointment they at first occasioned me*.” Scientific 
history can have few incidents more worthy. I do not know what 
is most striking in the story, the pathos or the moral dignity of 
Bentham’s attitude. 5 
Darwin necessarily restricted himself in the Origin to establishing 
the general principles which would account for the facts of distribu- 
tion, as a part of his larger argument, without attempting to illustrate 
them in particular cases. This he appears to have contemplated 
doing in a separate work. But writing to Hooker in 1868 he 
1 Origin, p. 360. 
2 «The Botanical Work of Darwin,” Ann. Bot. 1899, p. xi. 
3 More Letters, 1. p. 134. * Life and Letters, 11. p. 294, 
