PHYSICAL CONDITIONS— POSTGLACIAL. 517 



eighth parallel. The structure of the wood was found to differ 

 remarkably in its anatomical character from that of any other 

 conifer with which Dr. Hooker was acquainted, and the peculiar 

 conditions of an arctic latitude — long months of day, succeeded 

 by long months of night — seemed to him to afford an adequate 

 explanation of the appearance presented. 1 



Thus there are several lines of evidence which seem to lead 

 to one and the same conclusion — the testimony of the plants 

 supporting and confirming that of the marine mollusca. It will 

 be observed that the view of a postglacial warmer epoch does 

 not rest upon the former occurrence of one or two species far 

 north of their present range, of which we have even in our own 

 day some notable examples, but is based upon a much broader 

 foundation. And I am much strengthened in the opinion I 

 have been led to form by finding that Sir Joseph Hooker has 

 independently come to similar conclusions. Upon the publica- 

 tion of a short outline of the subject in a recent number of The, 

 Scottish Naturalist, Sir Joseph was good enough to write me as 

 follows : — " The case for a postglacial warmer epoch than the 

 present is, I think, fairly made out, though too much stress 

 cannot be laid upon the presence or absence of pine and other 

 trees, which in small areas may depend upon very local causes. 

 For instance, it is hard to say why certain parts of Scandinavia 

 are clothed with pine, others with fir, others with beech, and 

 still others with oak, quite irrespective of latitude and isother- 

 mals ; and there are considerable areas in South Scotland where 

 I am assured the Scotch fir cannot be induced to grow at all, 

 and where conifers of much warmer climates thrive. The dis- 

 tribution of forest trees (and other plants) is, for a considerable 

 distance towards their polar and tropical limits, exceedingly 

 capricious ; and where two or three species of trees of somewhat 

 similar powers of endurance co-exist, they are apt to replace one 

 another, so to speak, without any definite relation to the extreme 

 limits to which they are individually able to attain. One of 

 the most instructive results of Nares's Expedition was the evi- 



1 Op. cit. p. 381. 



