^4 SPREAD OF EVOLUTION. [1861. 



billions to an Insect Fauna of the Amazon Valley," in the 

 'Transactions of the Entomological Society/ vol. 5, N.S.* 

 Mr. Bates points out that with the return, after the glacial 

 period, of a warmer climate in the equatorial regions, the 

 "species then living near the equator would retreat north 

 and south to their former homes, leaving some of their con- 

 geners, slowly modified subsequently ... to re-people the 

 zone they had forsaken." In this case the species now living 

 at the equator ought to show clear relationship to the species 

 inhabiting the regions about the 25th parallel, whose distant 

 relatives they would of course be. But this is not the case, 

 and this is the difficulty my father refers to. Mr. Belt has 

 offered an explanation in his ' Naturalist in Nicaragua ' 

 (1874), p. 266. " I believe the answer is that there was much 

 extermination during the glacial period, that many species 

 (and some genera, &c., as, for instance, the American horse), 

 did not survive it .... but that a refuge was found for 

 many species on lands now below the ocean, that were un- 

 covered by the lowering of the sea, caused by the immense 

 quantity of water that was locked up in frozen masses on the 

 land."] 



C. Darwin to J. D. Hooker. 



Down, 27th [March 1861]. 



MY DEAR HOOKER, I had intended to have sent you 

 Bates's article this very day. I am so glad you like it. I 

 have been extremely much struck with it. How well he 

 argues, and with what crushing force against the glacial doc- 

 trine. I cannot wriggle out of it : I am dumbfounded ; yet 

 I do believe that some explanation some day will appear, and 

 I cannot give up equatorial cooling. It explains so much 

 and harmonises with so much. When you write (and much 

 interested I shall be in your letter) please say how far floras 

 are generally uniform in generic character from o to 25 N. 

 and S. 



* The paper was read Nov. 24, 1860. 



