ALPINE AND ARCTIC PLANTS 435 



the specimens from this old mummy were at once recognised 

 by Professor Tuckerman as identical with this species as it 

 occurs on the White Hills and on Katahdin, in Maine. These 

 facts are, however, easily explicable in comparison with those 

 that relate to the flowering plants. 



The spores of lichens and mosses float lighter than the light- 

 est down in the air, and may be wafted over land and sea, and 

 dropped everywhere to grow where conditions may be favour- 

 able. We can form an idea of this from the fact that the vol- 

 canic dust, consisting of shreds of pumice, etc., thrown up by 

 the eruption of Krakatoa, in 1 883, was wafted, in a day or two, 

 round the globe, and remained suspended for months in the 

 atmosphere. The spores of many cryptogamous plants are 

 even lighter than volcanic dust. Had the Egyptian embalmer 

 used some of the first created specimens of Evernia furfuracea, 

 it might easily, within the three thousand years or so since his 

 work was done, have floated round the world and established 

 itself on the White Hills. But, as we shall see, neither the time 

 nor means would suffice for the flowering plants. The only 

 available present agency for the transmission of these would be 

 in the crops or the plumage of the migratory birds ; and when 

 we consider how few of these, on their migrations from the 

 north, could ever alight on these hills, and the rarity of their 

 carrying seeds in a state fit to vegetate, and further, that few of 

 these plants produce fruits edible by birds, or seeds likely to 

 attach themselves to their feathers, the chances become infi- 

 nitely small of their transmission in this way. The most profit- 

 able course of investigation in this and most other cases of 

 apparently unaccountable geographical distribution, is to inquire 

 as to the past geological conditions of the region, and how 

 these may have affected the migrations of plants. 



The earlier geological history of these mountains far ante- 

 dates our existing vegetation. It belongs, in the first instance, 

 to the Archaean and early Palaeozoic period, in which the 



