Page:Popular Science Monthly Volume 75.djvu/380

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

This ancient circumpolar forest flourished at a time when the climate of the Arctic regions was almost warm temperate in its character 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, however, 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 individual 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 temperatures along the edge of the ice sheet, began to occupy the newly uncovered 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