EOCENE FERNS. 73 



sub-tropical plants are found. Erom a comparison of the relative size of recent pinnae 

 from different latitudes with those of Bournemouth and of Bovey, the difference in 

 temperature and consequently of elevation between the two stations might be gauged 

 the more safely from the persistency with which the average size is maintained at each 

 locahty. Want of heat evidently excluded it, like the Chrysodium,\x(sm. the Scotch, Irish, 

 and all Northern Eocenes, though in a stunted form it is now acclimatised in Kamschatka, 

 while the absence of both from the entire series of Swiss Tertiaries, shows that 

 unsuitable conditions continuously prevailed there. 



A third Fern, Onoclea Jiebraidica, from the basalt formation of Mull, is equally 

 identical with a living species. A discovery made during the progress of this work, 

 while still leaving the true age of this formation somewhat a matter of doubt, shows so 

 conclusively that the North British and Irish Tertiary floras are a continuation of the 

 Eocene floras to the south, that to omit them in this work would be to omit a clue 

 which may render possible the solution of the most interesting problems connected with 

 past and present plant distribution. 



The floras from the Eocenes below the London Clay are remarkably homogeneous in 

 England, and present relatively few species. They are, as long since pointed cut by Sir 

 J. Hooker in the case of the Reading plants, of remarkably temperate aspect, the leaves 

 and fruits of Platmms being conspicuous, among a number of undetermined prevaihng 

 types. This flora is sufficiently similar to that from the Greenland Tertiaries collected 

 by Mr. Why ni per, and different to other Tertiary floras, to show that a close relation- 

 ship must have existed between them. The inference from this is obviously that as 

 temperature increased during the London Clay period the old temperate indigenous 

 flora of our latitudes was driven as far north as Greenland, where it must have ren)ained 

 until a diminution of heat again enabled it to descend. The effect of the convergence 

 for so long a time, of perhaps very dissimilar floras from different longitudes towards the 

 land areas of the pole, consequent on a general rise of temperature in the northern 

 hemisphere in Eocene time, would be to mingle the floras of three continents ; so that 

 when they redescended, quantities of new forms would almost certainly appear in each 

 area. During nearly the entire Eocene period they were probably continuously modified 

 in the direction of existing trees, and in that sense species no doubt did originate near the 

 pole, as Saporta claims, and when they reoccupied the temperate latitudes of Europe it 

 was as a Miocene flora. The recognition of an Eocene temperate flora in our latitudes, 

 and consequently of a period when arctic conditions prevailed at the Pole similar to those 

 of the present day, explains the universal break in the sequence of floras between 

 Cretaceous and Tertiary, noticed all round the Arctic circle, and is also a considerable 

 step towards a comprehension of the past and the existing plant distribution of the northern 

 hemisphere. It fixes for the first time the ages of the Arctic Tertiary floras, and limits 

 the period during which they could possibly have grown there to between the London Clay 

 and the close of the Oligocene. The Mull and the Antrim floras are intermediate between 



