1848.] BUNBURY ON FOSSIL PLANTS. 137 



western Alps to retain its vegetation unchanged through so many- 

 geological periods, while that of the surrounding regions was under- 

 going repeated changes. 



Some few instances certainly are kno"VMi of the insulated occurrence 

 of tropical species, especially of ferns and Lycopodia, in temperate 

 regions, far beyond their ordinary geographical range ; as, for ex- 

 ample, Trichomanes radicans * in Ireland, and Lycopodium cernuum 

 in the Azores. But to find a parallel case to that under considera- 

 tion, we must suppose an island, or a small tract of country, in which 

 all the ferns were specifically different from those of the surround- 

 ing countries, and identical with those of some far-distant region ; 

 for, in such cases, distance in space may be considered as representing 

 distance in geological time. I know of nothing analogous to this in 

 the present state of things. 



Lastly, I must mention the hypothesis proposed by M. Adolphe 

 Brongniart. He holds, that the plants which we find preserved in 

 the slates of the Savoy Alps did not grow in those regions, but were 

 drifted from great distances ; that the peculiar vegetation which had 

 been widely spread over the globe in the carboniferous period, con- 

 tinued to exist in the hotter parts of the earth long after it had be- 

 come extinct in our temperate regions ; and that plants belonging to 

 those hotter climates were occasionally drifted as far north as where 

 the Alps now exist, and buried in the deposits in which wt find them 

 alternating with belemnites. This is perhaps the most plausible ex- 

 planation that has been offered, and yet there seem to be some con- 

 siderable difiiculties in the way of it. Although the vegetable remains 

 in the Alpine slates are not in general very well preserved, yet their 

 condition is not so very different from that of ordinary coal-plants as 

 it would seem that, according to this theory, it ought to be ; and it is 

 not easy to conceive how the delicate leaves of ferns could be drifted, 

 either by the sea or by rivers, for so great a distance as from a tropi- 

 cal to a temperate climate, without being so much damaged as to lose 

 all their distinctive characters. Fruits, and seeds, and branches of 

 tropical plants are occasionally^ wafted by the sea to our coasts ; but 

 I never heard of any instance of leaves being so conveyed. Nor is it 

 easy to understand, on this supposition, how it happened that these 

 tropical ferns were not drifted to other parts of Europe, besides the 

 district under consideration. 



Nevertheless, I must acknowledge that I have no more satisfactory 

 explanation to offer ; nor do I see any way out of the diflSculty, un- 

 less by adopting the opinion of M. jNIichelin, to which I have already 

 referred. My attention has been directed chiefly to the botanical 

 aspect of the question, and I must leave the farther discussion of it 

 to those more versed than myself in strictly geological investigations, 

 and especially to those who have had experience in the difiicult re- 

 searches of Alpine geology. 



* See Hooker's * Species Filicum.' 



