1870-18S2] CONDITIONS OF EXISTENCE 391 



To K. Semper. 1 Letter 303 



Down, Feb. 6th, 1881. 



Owing to all sorts of work, I have only just now finished 

 reading your Nat. Conditions of Existence? Although a book 

 of small size, it contains an astonishing amount of matter, and 

 I have been particularly struck with the originality with which 

 you treat so many subjects, and at your scrupulous accuracy. 

 In far the greater number of points I quite follow you in your 

 conclusions, but I differ on some, and I suppose that no two 

 men in the world would fully agree on so many different 

 subjects. I have been interested on so many points, I can 

 hardly say on which most. Perhaps as much on Geographical 

 Distribution as on any other, especially in relation to M. 

 Wagner. (No ! no ! about parasites interested me even more.) 

 How strange that Wagner should have thought that I meant 

 by struggle for existence, struggle for food. It is curious that 

 he should not have thought of the endless adaptations for the 

 dispersal of seeds and the fertilisation of flowers. 



Again I was much interested about Branchipus and 

 Artemia? When I read imperfectly some years ago the 

 original paper I could not avoid thinking that some special 

 explanation would hereafter be found for so curious a case. 

 I speculated whether a species very liable to repeated and 

 great changes of conditions, might not acquire a fluctuating 

 condition ready to be adapted to either conditions. With 

 respect to Arctic animals being white (p. 116 of your book) it 

 might perhaps be worth your looking at what I say from 

 Pallas' and my own observations in the Descent of Man (later 

 editions) Ch. VIII., p. 229, and Ch. XVIII, p. 542. 



I quite agree with what I gather to be your judgment, 

 viz, that the direct action of the conditions of life on 



1 Karl Semper (1832-93), Professor of Zoology at Wiirzburg. He is 

 known for his book of travels in the Philippine and Pelew Islands, for 

 his work in comparative embryology, and for the work mentioned in the 

 above letter. See an obituary noticein Nature, July 20th, 1893, p. 271. 



2 Semper's Natural Conditions of Existence as they affect Animal Life 

 (Internat. Sci. Series), 1881. 



3 The reference is to Schmankewitsch's experiments, p. 158 : he kept 

 Artemia salina in salt-water, gradually diluted with fresh-water until it 

 became practically free from salt ; the crustaceans gradually changed in 

 the course of generations, until they acquired the characters of the genus 

 Branchipus. 



