THE DIFFERENTIATION OF SPECIES. 77 



Professor Alexander Agassiz ^ has ridiculed the idea of the 

 continental origin of the Galapagos. I have asked him to 

 explain the harmonic distribution of the fauna and flora on the 

 elevation theory.^ He nor anybody else so far has been able 

 to explain this harmonic distribution by accidental immigration 

 to the islands; which we have to assume of, course on that 

 theory. 



I do not see any difficulty in accepting the theory of sub- 

 sidence ; the I 500-fathom line probably embraces Cocos Island 

 and the Galapagos from Central America (Colombia), and 1 500 

 fathoms and more are even admitted now by Wallace ^ as no ob- 

 jection to the continental origin of an island. He says : "All that 

 is necessary to maintain is, that existing continents with their 

 included seas and their surrounding oceanic waters as far as 

 the 1 500-fathom, or in some extreme cases, the 2000-fathom 

 line, mark out the areas within which the continental lands of 

 the globe have been built up ; while the oceanic areas beyond 

 the 2000-fathom line have almost certainly been ocean through- 

 out all known geological time." {Natural Science, August, 

 1892.) But even this is, I think, quite arbitrary. There is 

 very little doubt that during the Jurassic Africa was connected 

 with South America, and in this case even 2000 fathoms would 

 be not sufficient. The opinion expressed by Jukes-Brown * in 

 the September number of Natural Science, 1892, seems to me 

 perfectly sound. He says : " Those who oppose the doctrine 

 of permanence say that the present continents are the outcome 

 of a long series of geographical mutations, and I would add that 

 each phase was an episode in a long process of geographical 



1 Agassiz, Alexander : General sketch of the expedition of the " Albatross," 

 from February to May, 1891. Bull. Mus. Contf. Zool., vol. XXIII, No. i, 1892, 

 pp. 70-74. W. Botting Hemsley also ridicules my idea, quoting Agassiz in a late 

 paper published in Scientific Progress. London, vol. I, No. 5, July, 1894, pp. 400, 

 401, "Insular Floras "; but he also does not try to explain the harmonic distri- 

 bution. 



2 Baur, G. : Professor Alexander Agassiz on the origin of the Fauna and Flora 

 of the Galapagos Islands. Science, vol. XIX, 1892, p. 176. 



3 Wallace, A. Russel : The Permanence of the Great Oceanic Basins. Nat. 

 Science, August, 1892, vol. I. 



* Jukes-Brown, A. J.: The evolution of Oceans and Continents. Nat. Science, 

 September, 1892, vol. I, pp. 508-513. 



