ok to base any statement
on a secure foundation of evidence, and for this the world, till the
publication of his letters, could not do him justice. He was a great
admirer of Herbert Spencer, whose "prodigality of original thought"
astonished him. "But," he says, "the reflection constantly recurred to
me that each suggestion, to be of real value to service, would require
years of work." (Ibid. II. page 235.)

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." ("Origin", page 360.) In this single
sentence Darwin has stated a theory which, as his son F. Darwin has said
with justice, has "revolutionized botanical geography." ("The Botanical
Work of Darwin", "Ann. Bot." 1899, page xi.) 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" ("More Letters", I. page 134.) 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