PHYSICAL CONDITIONS— PLEISTOCENE. 335 



flora embraced Scots fir, spruce fir, yew, alder, oak, birch, 

 white and yellow water-lilies, bog-bean, common sloe, etc. — 

 indicating a climate perhaps a little colder, but not essentially 

 differing from that of Norfolk at present. But as the plants are 

 traced upwards through the strata they were found by Mr. 

 Nathorst to become more and more stunted and meagre, until 

 in a bed immediately underlying the boulder-clay he came upon 

 the Arctic willow (Salix polaris), and a moss (Hypnum ttirgescens) 

 now confined in temperate latitudes to the highest alps. 1 The 

 latter grows in Herjedalen on the Dovrefield, in the north of 

 Scandinavia, in Bear Island, Spitzbergen, and Greenland. Tims 

 we learn how the arctic and alpine flora, driven southwards by 

 the encroachment of the great northern mer de glace, at last 

 came to occupy the low grounds of temperate Europe. 



I shall not attempt here to summarise the history, so far as 

 that has been ascertained, of each particular glacial and inter- 

 glacial epoch, but after some general remarks will proceed to 

 describe the climatic and geographical changes which charac- 

 terised our continent towards the close of the Pleistocene, that 

 is to say during the last interglacial and glacial epochs. That 

 the successive invasions of Northern Europe by vast mers de 

 glace, and the repeated appearance, in the hilly regions of more 

 southern latitudes, of perennial snow-fields, must have profoundly 

 modified the flora and fauna cannot be doubted. It is highly 

 probable also that the presence of the Mediterranean, by present- 

 ing a more or less insurmountable barrier to the south, and thus 

 cutting off the retreat of many species, may have contributed 

 largely to the extinction of Pliocene animals and plants, which 

 but for that barrier might have crossed into Africa, and found 

 there conditions suited to their needs. It is for this reason that 

 the European flora of to-day differs so much more from that of 

 Pliocene and Miocene times, than the flora of North America 

 does from the plant-life of those periods. In the latter continent 

 there existed a continuous passage to the south, across which 

 plants and animals alike could make good their retreat. But 



1 Antiquity of Man, p. 261 ; Ofversigt of Kongl. Vet.-Akad. Fork., 1873, p. 18. 



