428 GEOGRAPHICAL DISTRIBUTION [CHAP. VI 



Letter 327 Glacial epoch, and the consequent migration of organisms 

 through the tropics. There are a good many difficulties, but 

 upon the whole it explains much. This has been a favourite 

 notion with me, almost since I wrote on erratic boulders of 

 the south. It harmonises with the modification of species ; 

 and without admitting this awful postulate, the Glacial epoch 

 in the south and tropics does not work in well. About 

 Atlantis, I doubt whether the Canary Islands are as much 

 more related to the continent as they ought to be, if formerly 

 connected by continuous land. 



Hooker, with whom I have formerly discussed the notion 

 of the world or great belts of it having been cooler, though he 

 at first saw great difficulties (and difficulties there are great 

 enough), I think is much inclined to adopt the idea. With 

 modification of specific forms it explains some wondrous odd 

 facts in distribution. 



But 1 shall never stop if I get on this subject, on which I 

 have been at work, sometimes in triumph, sometimes in 

 despair, for the last month. 



Letter 328 Asa Gray to C. Darwin. 



Received August 2Oth, 1856. 



I enclose you a proof of the last page, that you may see 

 what our flora amounts to. The genera of the Cryptogams 

 (Ferns down to Hepaticae) are illustrated in fourteen crowded 

 plates. So that the volume has become rather formidable as 

 a class-book, which it is intended for. 



I have revised the last proofs to-day. The publishers will 

 bring it out some time in August. Meanwhile, I am going to 

 have a little holiday, which I have earned, little as I can spare 

 the time for it. And my wife and I start on Friday to visit 

 my mother and friends in West New York, and on our way 

 back I will look in upon the scientific meeting at Albany on 

 the 2Oth inst, or later, just to meet some old friends there. 



Why could not you come over, on the urgent invitation 

 given to European savans and free passage provided back 

 and forth in the steamers ? Yet I believe nobody is coming. 

 Will you not come next year, if a special invitation is sent 

 you on the same terms ? 



Boott lately sent me your photograph, which (though not 

 a very perfect one) I am well pleased to have. . . . 



