230 EVOLUTION OF PLANTS 



as revealed by their fossil remains, was evidently very 

 different from that of the present time. The most strik- 

 ing point about these Tertiary fossils is the wide distri- 

 bution of many types now extremely limited in their 

 range, and a careful study of the question leads inevi- 

 tably to the conclusion that at this period in the earth's 

 history a very uniform climate must have prevailed 

 over a large part of the northern hemisphere, and cor- 

 responding to this there was a very similar flora through- 

 out its whole extent. It is also evident that a very 

 much warmer and more even temperature must have 

 prevailed in the higher latitudes which at present are 

 incapable of supporting any but the hardiest plants. In 

 early Tertiary times palms, sequoias, magnolias, sassa- 

 fras, tulip-trees, and many other types, now quite absent 

 from these regions, were abundant in northern Europe, 

 and even in Greenland and Siberia, showing conclusively 

 that at that time a very much milder climate'must have 

 prevailed there than at present. These same types occur 

 fossil in the arid western United States, from which they 

 have completely disappeared, owing, no doubt to the un- 

 favorable conditions now existing. 



In the higher latitudes at the present day, there 

 exists a zone extending round the earth, where the cli- 

 matic conditions are very uniform, and where the plants 

 are very similar throughout, much as was the case 

 over a much wider zone in Tertiary times ; but in- 

 stead of laurels and magnolias, palms and sequoias, 

 we find firs and willows, birches and aspen-poplars. 

 Many northern plants, like the beautiful little Linnsea 

 and white birch, are equally at home in Norway and 

 Canada, and the reasons are the same which governed 



