436 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



In all these comparative statements I omit tlie Stigmaria ficoides, 

 because it is very doubtful whetiier it can be considered as a species, 

 in any proper sense of tbe v/ord ; and if it really be, as there seems 

 great reason to believe, merely tlie root of one or more kinds of Sigil- 

 laria, it is evidently unavailable for such comparisons. 



Among the fossil plants of the coal-formation, the Ferns alone can 

 be referred with certainty to their proper place in the system of the 

 vegetable world, and approach near enough to existing types to admit 

 of satisfactory comparison. It may therefore be worth while to pay 

 some attention to the geographical distribution of recent Ferns. 



Looking, in the first place, to Europe, we find much less difference 

 between the Ferns of the northern and those of the southern parts of 

 this continent, than if we compare the flowering plants of those 

 regions. Thus, of 42 Ferns found in Northern Italy*, 8 only are 

 strangers to Britain. Iceland, according to Sir William Hooker, has 

 14 Ferns, all of which are found in England, and all but one in Italy. 

 Of 26 Ferns enumerated as natives of Greece f, 21 are British, and 

 8 common to Greece and Iceland. Even beyond the limits of Eu- 

 rope we find many of the plants of this tribe, which are most familiar 

 to us at home, reappearing in the islands of the Atlantic, almost on 

 the verge of the tropics. One-third of the Ferns of Madeira J (12 

 species out of oQ) are British. Two species {Polypodium vulgare 

 and Cyst opt eris frayilis) are common to Madeira and Iceland, — a 

 greater range in latitude than any fossil Fern has yet been ascer- 

 tained to possess. In the Azores, — less removed from us indeed in 

 latitude, but separated by a wide expanse of ocean, — the proportion 

 of identical species is much greater ; more than half the Ferns of the 

 Azores § (16 out of 28) being natives of Britain. 



When we cross the Atlantic, indeed, a greater amount of dif- 

 ference is found in this tribe of plants. Out of 69 Ferns of North 

 America, enumerated by Pursh, only 13 are considered by him as 

 identical with European species, and in some of these cases the iden- 

 tity is very doubtfid. Those which are really common to the tv/o 

 continents belong chiefly to the northern parts of America, and among 

 them are most of those which, in Europe, have the widest range, 

 and are most indiiferent to temperature and exposure. 



It is probable that the great difference in this part of the Floras of 

 Europe and North America may be owing not merely to the breadth 

 of sea between them, but perhaps more to the diiference of their 

 climates, that of Europe having comparatively an insular, and that 

 of America an extreme character. If the United States had a climate 

 resembling that of the west of Europe, it is probable that a much 

 larger proportion of Ferns would be common to the two continents. 



Within the tropics, many plants of this tribe have very extensive 

 ranges both in latitude and longitude. I have not the means of 

 drawing up full comparative statements of these, but it is certain that 



* PoUini, Flora Veronensis. 



t Smith and Sibthorp, Prodromus FL Grsecse. 



% Holl and Lowe, in Hooker's Journal of Botany, vol. i. 



§ Watson, in Hook. Lond. Journ. Bot., vol. iii. 



