APHORISMS AND REFLECTIONS 67 



by the " northern outliers " ; just as the vegetation 

 of the land, down to the sea-level, had the northern 

 character which is, at present, exhibited only by the 

 plants which live on the tops of our mountains. 

 But, as the glacial epoch passed away, and the 

 present climatal conditions were developed, the 

 northern plants were able to maintain themselves 

 only on the bleak heights, on which southern forms 

 could not compete with them. And, in like manner, 

 Forbes suggested that, after the glacial epoch, the 

 northern animals then inhabiting the sea became 

 restricted to the deeps in which they could hold 

 their own against invaders from the south, better 

 fitted than they to flourish in the warmer waters of 

 the shallows. Thus depth in the sea corresponded 

 in its effect upon distribution to height on the 

 land. 



Among the scientific instructions for the voyage J 

 drawn up by a committee of the Royal Society, 

 there is a remarkable letter from Von Humboldt 

 to Lord Minto, then First Lord of the Admiralty, 

 in which, among other things, he dwells upon 

 the significance of the researches into the micro- 

 scopic composition of rocks, and the discovery 

 of the great share which microscopic organisms 

 take in the formation of the crust of the earth at 

 the present day, made by Ehrenberg in the years 

 1836-39. Ehrenberg, in fact, had shown that the 

 extensive beds of "rotten-stone" or "Tripoli" 

 which occur in various parts of the world, and 

 notably at Bilin in Bohemia, consisted of accumu- 

 lations of the silicious cases and skeletons of 

 DiatomacetE, sponges, and Radiolaria ; he had proved 

 that similar deposits were being formed by Diato- 

 inaccce, in the pools of the Thiergarten in Berlin and 



1 Of the Challenger. 



