436 THE UNFINISHED BOOK. [1856. 



Azores, or for Hooker's continent ; I believe Hooker does not 

 infer a continuous continent, but approximate groups of 

 islands, with, if we may judge from existing continents, not 

 profoundly deep sea between them ; but the argument from 

 the volcanic nature of nearly every existing oceanic island 

 tell against such supposed groups of islands, — for I presume 

 he does not suppose a mere chain of volcanic islands belting 

 the southern hemisphere. 



Fourthly. The supposed continental extensions do not 

 seem to me, perfectly to account for all the phenomena of 

 distribution on islands ; as the absence of mammals and 

 Batrachians ; the absence of certain great groups of insects 

 on Madeira, and of Acacise and Banksias, &c., in New Zea- 

 land ; the paucity of plants in some cases, &c. Not that 

 those who believe in various accidental means of dispersal, 

 can explain most 'of these cases ; but they may at least say 

 that these facts seem hardly compatible with former continu- 

 ous land. 



Finally. For these several reasons, and especially consider- 

 ing it certain (in which you will agree) that we are extremely 

 ignorant of means of dispersal, I cannot avoid thinking that 

 Forbes' 'Atlantis,' was an ill-service to science, as checking 

 a close study of means of dissemination. I shall be really 

 grateful to hear, as briefly as you like, whether these argu- 

 ments have any weight with you, putting yourself in the 

 position of an honest judge. I told Hooker that I was going 

 to write to you on this subject ; and I should like him to read 

 this ; but whether he or you will think it worth time and post- 

 age remains to be proved. 



Yours most truly, 



Charles Darwin. 



[On July 8th he wrote to Sir Charles Lyell. 



*' I am sorry you cannot give any verdict on Continental 

 extensions ; and I infer that you think my argument of not 

 much weight against such extensions. I know I wish I could 

 believe so."] 



