94 STUDIES, SCIENTIFIC AND SOCIAL CHAP. 



to form permanent ponds or lakes, by which means, 

 combined with the natural reservoirs already alluded to, 

 and aided by the check to evaporation which additional 

 planting will produce, the forest itself and even the 

 surrounding country will be permanently benefited. By 

 extensive draining, on the other hand, water is carried 

 away rapidly from the district, and with it much fertilising 

 matter; the climate is made drier, and the growth of 

 herbage as well as of trees and shrubs is rendered less 

 luxuriant. 



Differences of Temperate Forests, and their Causes. 



Coming back now to the general question of forest 

 distribution in the Northern Hemisphere, many of my 

 readers must have been struck by the singular inequality 

 and remarkable contrasts of the four great temperate 

 forests of which we have proposed that illustrations should 

 be grown at Epping. In a lecture recently delivered before 

 the Harvard University Natural History Society, Pro- 

 fessor Asa Gray has given an explanation of these 

 contrasts, which will commend itself to all naturalists who 

 know how important has been the agency of the glacial 

 period in bringing about the existing relations between 

 Alpine and Arctic plants. 



Let us first consider the remarkable difference between 

 the forest vegetation of Eastern America and that of 

 Europe and Western Asia. The latter area is the more 

 extensive and more varied of the two, yet its trees, both 

 deciduous and coniferous, are scarcely half as numerous 

 or half as diversified. Why, we naturally ask, is America 

 so rich ? Professor Asa Gray answers, it is not America 

 that is exceptionally rich, but Europe that is exceptionally 

 poor. This is shown in two ways. Firstly, because 

 America, rich as it is, is surpassed by Eastern Asia ; and, 

 secondly, because Europe itself was formerly at least as 

 rich as America is now. During the later Miocene or 

 Pliocene periods, Europe possessed most of the generic 

 groups of trees now confined to North America and East 

 Asia, and was wonderfully rich in different kinds. The 



