1 844.] MUTABILITY OF SPECIES. 29 



have unclearly written, I would not have troubled you with 

 the reading of it. Believe me, at last not hypothetically, 



Yours very sincerely, 



C. Darwin. 



C. Darwin to J. D. Hooker. 



Down, 1844. 



... I forget my last letter, but it must have been a very 

 silly one, as it seems I gave my notion of the number of 

 species being in great degree governed by the degree to 

 which the area had been often isolated and divided ; I must 

 have been cracked to have written it, for I have no evidence, 

 without a person be willing to admit all my views, and then 

 it does follow ; but in my most sanguine moments, all I 

 expect, is that I shall be able to show even to sound Natur- 

 alists, that there are two sides to the question of the immut- 

 ability of species ; that facts can be viewed and grouped 

 under the notion of allied species having descended from 

 common stocks. With respect to books on this subject, I 

 do not know of any systematical ones, except Lamarck's, 

 which is veritable rubbish ; but there are plenty, as Lyell, 

 Pritchard, &c, on the view of the immutability. Agassiz 

 lately has brought the strongest argument in favour of immut- 

 ability. Isidore G. St. Hilaire has written some good Essays, 

 tending towards the mutability-side, in the ' Suites a Buffon,' 

 entitled " Zoolog. Generale." Is it not strange that the author 

 of such a book as the ' Animaux sans Vertebres ' should 

 have written that insects, which never see their eggs, should 

 will (and plants, their seeds) to be of particular forms, so as 

 to become attached to particular objects. The other common 

 (specially Germanic) notion is hardly less absurd, viz. that 

 climate, food, &c., should make a Pediculus formed to climb 

 hair, or wood-pecker to climb trees. I believe all these 

 absurd views arise from no one having, as far as I know, 

 approached the subject on the side of variation under domest- 



