1858.] CLIMATE AND MIGRATION. 491 



sea, and then came right in at right angles to the shore ! One 

 day such a number of insects were washed up by the tide, 

 and I brought to life thirteen species of Coleoptera ; not that 

 I suppose these came from France. But do you watch for 

 thistle-seed as you saunter along the coast. . . . 



C. Darwin to Asa Gray. 



Aug. nth [1858]. 



My dear Gray, — Your note of July 27th 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 in- 

 valuable assistance 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 be- 

 fore 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, judg- 

 ing from shells, what they are now. At this period when 

 all animals and plants ranged io° 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 



