376 THE POPULAR SCIENCE MONTHLY 



poplars and oaks — all of which are nearly related to the modern types. 1 

 This ancient circumpolar forest flourished at a time when the 

 climate of the Arctic regions was almost warm temperate in its char- 

 acter and capable of supporting a rich and varied tree-life throughout 

 an immensely long period of time. Toward the close of this period 

 the increasing cold which culminated in a glacial epoch caused a 

 gradual change in the forest conditions. The more northerly portion 

 of this wide-spread forest was not able to survive the change, its species 

 were forced out of the region, finding more suitable conditions in lands 

 farther to the south, where some, undoubtedly, had already established 

 themselves. A large number of species, like the oaks and beeches and 

 some of the conifers easily adapted themselves to a varied environment 

 and spread widely around the north temperate zone. Others, how- 

 ever, found suitable conditions of life only in certain localities, often 

 widely apart, but with similar climatic features. Here we have a 

 solution of the similarity of tree forms in forests so widely separated 

 geographically as those of eastern North America and eastern Asia. 

 In every other portion of the wide region into which the trees of this 

 polar forest migrated a certain number of species failed to occupy the 

 soil through some adverse conditions in climate or life relations. Cut 

 off from their center of development in the polar area, they spread to 

 the south, but only in two localities did they succeed in establishing 

 themselves — on the eastern side of the two great northern land masses, 

 where almost similar climatic influences prevail. 



It is of more than passing interest to thus trace back the history 

 of our forests. As with men so with trees. Countless generations 

 succeed one another, occupying the soil in which the remains of older 

 generations lie buried, each new generation springing from the one 

 before it and bearing the marks of an inheritance that allies its indi- 

 vidual members with other men and trees in far distant lands, and that 

 carries them back through a seemingly endless chain of life to a remote 

 antiquity. 



With the retreat of the ice at the close of the last glacial epoch, 

 and the gradual assumption of the present climate and physical features 

 of eastern North America, the more hardy coniferous trees, and some 

 of the broad-leaved species that had adapted themselves to low tempera- 

 tures along the edge of the ice sheet, began to occupy the newly uncov- 

 ered land as a northern belt of pine forest. This expansion of tree 

 life toward the north must have relieved the long overcrowded southern 

 area and permitted a fuller development of the summer-green, broad- 

 leaved forest with its great variety of forms. The deciduous habit of 

 this summer-green forest is clearly an adaptation to a low winter 



1 " The Relations of North American to North East Asian and Tertiary 

 Vegetation " — being portion of an address by Dr. Asa Gray, published as 

 Article V. in " Darwiniana." 



