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 vory 
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 Linnea 
and white birch, are equally at home in Norway and 
Canada, and the reasons are the same which governed 
