1858.] CLIMATE AND MIGRATION. 135 



C. Darwin to Asa Gray. 



Aug. nth [1858]. 



MY DEAR GRAY, Your note of 7uly 2/th has just reached 

 me in the Isle of Wight. It is a real and great pleasure to me 

 to write to you about my notions ; and even if it were not so, 

 I should be a most ungrateful dog, after all the invalu- 

 able assistance which you have rendered me, if I did not do 

 anything which you asked. 



I have discussed in my long MS. the later changes of 

 climate and the effect on migration, and I will here give you 

 an abstract of an abstract (which latter I am preparing of 

 my whole work for the Linnean Society). I cannot give 

 you facts, and I must write dogmatically, though I do not 

 feel so on any point. I may just mention, in order that you 

 may believe that I have some foundation for my views, that 

 Hooker has read my MS., and though he at first demurred to 

 my main point, he has since told me that further reflection 

 and new facts have made him a convert. 



In the older, or perhaps newer, Pliocene age (a little 

 before the Glacial epoch) the temperature was higher ; of 

 this there can be little doubt ; the land, on a large scale, 

 held much its present disposition : the species were mainly, 

 judging from shells, what they are now. At this period 

 when all animals and plants ranged 10 or 15 nearer the 

 poles, I believe the northern part of Siberia and of North 

 America, being almost continuous, were peopled (it is quite 

 possible, considering the shallow water, that Behring Straits 

 were united, perhaps a little southward) by a nearly uniform 

 fauna and flora, just as the Arctic regions now are. The 

 climate then became gradually colder till it became what 

 it now is ; and then the temperate parts of Europe and 

 America would be separated, as far as migration is concerned, 

 just as they now are. Then came on the Glacial period, 

 driving far south all living things ; middle or even southern 



