18431882] ARCTIC ALPINE PLANTS 439 



able to show very clearly in this case) in such change, I Letter 334 

 think, the result would be as follows. Some of the warm- 

 temperate forms would penetrate the Tropics long before 

 the sub-arctic, and some might get across the equator long 

 before the sub-arctic forms could do so (i.e. always supposing 

 that the cold came on slowly), and therefore these must have 

 been exposed to new associates and new conditions much 

 longer than the sub-arctic. Hence I should infer that we 

 ought to have in the warm-temperate S. hemisphere more 

 representative or modified forms, and fewer identical species 

 than in comparing the colder regions of the N. and S. I 

 have expressed this very obscurely, but you will under- 

 stand, I think, what I mean. It is a parallel case (but 

 with a greater difference) to the species of the mountains 

 of S. Europe compared with the arctic plants, the S. 

 European alpine species having been isolated for a longer 

 period than on the arctic islands. Whether there are many 

 tolerably close species in the warm-temperate lands of the 

 S. and N. I know not ; as in La Plata, Cape of Good Hope, 

 and S. Australia compared to the North, I know not. I 

 presume it would be very difficult to test this, but perhaps 

 you will keep it a little before your mind, for your argu- 

 ment strikes me as by far the most serious difficulty which 

 has occurred to me. All your criticisms and approvals are 

 in simple truth invaluable to me. I fancy I am right in 

 speaking in this note of the species in common to N. and S. 

 as being rather sub-arctic than arctic. 



This letter does not require any answer. I have written 

 it to ease myself, and to get you just to bear your argument, 

 under the modification point of view, in mind. I have had 

 this morning a most cruel stab in the side on my notion of 

 the distribution of mammals in relation to soundings. 



J. D. Hooker to C. Darwin. Letter 335 



Kew, Sunday [Nov. 1856]. 



I write only to say that I entirely appreciate your answer 

 to my objection on the score of the comparative rareness of 

 Northern warm-temperate forms in the Southern hemi- 

 sphere. You certainly have wriggled out of it by getting 

 them more time to change, but as you must admit that the 

 distance traversed is not so great as the arctics have to 



