28 CHARLES DARWIN. 



separate islands being rendered extremely rare from 

 the depth of the 1 sea, the direction of the currents, 

 and the absence of gales. In this way time for 

 specific modification was provided before the parti- 

 ally modified form could interbreed with the parent 

 species and thus lose its own newly - acquired 

 characteristics. 



Although Darwin made these observations on the 

 Beagle, they required, as Huxley has suggested 

 (Obituary [1888], " Darwiniana " : Collected Essays, 

 vol. ii., pp. 274-275. London, 1893), careful and 

 systematic working out before they could be trusted 

 as a basis on which to speculate ; and this could not 

 be done until the return home. The following letter 

 written by Darwin to Dr. Otto Zacharias in 1877 

 confirms this opinion. It was sent to Huxley by 

 Francis Darwin, and is printed in " Darwiniana" 

 (I c., p. 275) :- 



" When I was on board the ' Beagle,' I believed in the 

 permanence of species, but, as far as I can remember, vague 

 doubts occasionally flitted across my mind. On my return 

 home in the autumn of 1836, I immediately began to prepare 

 my journal for publication, and then saw how many facts 

 indicated the common descent of species, so that in July, 1837, 

 I opened a note-book to record any facts which might bear on 

 the question. But I did not become convinced that species 

 were mutable until I think two or three years had elapsed." 



It is interesting to note that both the lines of 

 evidence which appealed to Darwin so strongly, 

 point to evolution, but not to any causes of 

 evolution. The majority of mankind were only con- 



