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 wth justice, has "revolutionized botanical geography 2." It 

 explains how jihysical 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 ovm had been set do^vn 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, "I... 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. 



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 



' Origin, p. SCO. 



» "The Botauiciil Work of Darwin," Ann. Bot. 1809, p. xi. 



* More Letters, i. p. 134. * Life and Letters, ii. p. 204. 



