2 BRITISH EOCENE FLORA. 



the Greenland plants in Mr. Whymper's collection, and which rarely occur in any other 

 Tertiary floras, to show a close relationship. One inference from this is obviously that 

 when the warmer Eocene period set in during the formation of the London Clay the 

 older and more temperate Eocene flora was driven far north, where it probably remained 

 until a gradual diminution of heat enabled it to come south again. The effect of the 

 convergence on the land-areas of the Pole for so long a time of, perhaps, very dissimilar 

 floras from different longitudes, consequent on this rise of temperature in the northern 

 hemisphere in Eocene time, might naturally lead to a mingling to some extent of the 

 floras of the three northern continents, so that they might have redescended enriched 

 with a quantity of new forms. During all this lapse of time they were probably 

 becoming modified in the direction of existing trees, and in that sense no doubt species 

 did originate near the Pole as Saporta claims. When they reoccupied the temperate 

 latitudes of Europe they constituted the Miocene flora. The discovery, which I have 

 abundantly confirmed, of an Eocene temperate flora in the south of England — and conse- 

 quently of a period when arctic conditions were probably not greatly dissimilar to those 

 of the present — fully explains the break in the sequence between Cretaceous and Tertiary 

 floras noticed all round the Arctic Circle ; and is a considerable step towards a compre- 

 hension of the past, and even existing, plant distribution of the northern hemisphere, and 

 one therefore whose ultimate bearings can as yet only be shadowed forth. The migrations, 

 comparatively simple with regard to temperate floras, are infinitely complicated when 

 those of the sub-tropics, which replaced them in Europe, are considered. The causes 

 which led to the association together of types now characteristic of widely separated 

 latitudes and longitudes in one area during the Middle Eocene must have been very 

 varied, and seem, as yet, not to admit of any entirely satisfactory explanation. 



These types, it is evident, are now to a great extent represented in areas as widely 

 separated as tropical America, North-East Australia, and the East coast of Asia and 

 adjacent isles. The plants from Mull and Antrim, described by some as ]\liocene, 

 might, like those nearer the Pole, belong to any age between the beginning and the 

 end of the warmer Eocene period ; but they cannot be of earlier, and are unlikely to be of 

 later date. Whether, and to what extent, the Cretaceous floras of Greenland and 

 Dakota are connected with the older temperate Eocene floras, and what relationship the 

 Aix-la-Chapelle flora bears to them and to still older Cretaceous floras, is as yet quite 

 unknown and presents problems of deep interest.' 



1 Towards the practical solution of such questions I have, during twelve years, collected plants from 

 the British Eocenes. The results, although considerable, might have been greater had I found it possible 

 to utilise assistance to any extent, but my experience has been that collections of fossil plants lose the 

 greater part of their value when not directly made by those actually engaged in their study. I have 

 further procured, where possible, collections from deposits of other countries, such as Sezanne, Gelinden, 

 Cereste, and the Paris Basin, places whose Tertiary floras seemed likely to assimilate to those of Britain. 

 Many collections of Tertiary plants have of late years come into the possession of the British Museum, and 

 will, I hope, shortly be available for comparison. 



