374 THE POPULAR SCIENCE MONTHLY 



tion, like the hemlock, are found growing along the higher land and in 

 cool ravines as far south as the Lower Delaware Valley. 



The more familiar trees, however, mingle over a wide area of 

 country — southern New England and the Middle Atlantic district — 

 a transition region that lies between the northern coniferous forest 

 and the broad-leaved, summer-green forest of the great interior valley 

 and south Atlantic slope. 



Something besides temperature appears to control the distribution 

 of certain trees. Since the settlement of the country numerous species, 

 the natural habitats of which are far to the south, have been planted 

 and grown successfully in more northern localities. The catalpa, the 

 sourwood and the several species of magnolia are illustrations of this. 

 Just what is the determining factor in preventing such trees from 

 spreading northward (or others from spreading southward) it is diffi- 

 cult to say. Possibly some subtle condition in the balance of nature 

 — some item in the struggle for existence, has helped or hindered the 

 spread of certain trees, excluding some from localities already occupied 

 by others. It is well known that where pines have been cut off certain 

 species of oaks will spring up and occupy the land. The oak seedlings 

 must have been abundantly scattered over the soil for a long period 

 of time. Only a comparatively few species are enabled to hold their 

 own through some superior advantage in adaptation and grow up to 

 form a forest. A vast number must of necessity lie dormant in the 

 soil for centuries. In Denmark, since glacial times, there has evidently 

 been a succession of forests — oak following fir and beech following 

 oak through a period of many thousands of years. The varied relations 

 of the different species of trees to light, heat, moisture and soils, to 

 animals, and to other trees and plants, are involved in slow, deep-seated 

 processes, the expression of which is the forest as we see it. Our point 

 of view, however, is but momentary in the vast events of nature — a 

 fleeting glimpse, merely, of one picture in an endless biograph. 



The question of the succession of forests suggests another question 

 — that of the origin of the present Atlantic forest and its relation to 

 other forests in different parts of the world. In a general survey of 

 the forest trees of eastern North America there appears a large number 

 of types which are common also to Europe. With the exception of 

 the bald cypress and the hemlocks all the other coniferous types — pines, 

 spruces, firs and larches — are represented in Europe by closely allied 

 species. This is true also of a number of the deciduous trees — the 

 oak, beech, chestnut, elm, willow, birch, aspen, walnut, ash, maple, 

 plane tree, linden or basswood, and others are represented on both sides 

 of the Atlantic by more or less nearly related forms. This is not sur- 

 prising when we come to consider that all of the above-named trees 

 are decidedly northern in their distribution, and exist under very 

 similar climatic conditions. This is especially true of the coniferous 



