5 i8 THE POPULAR SCIENCE MONTHLY. 



limited in their northern extension by heat alone, we shall find many- 

 anomalies difficult to reconcile, as no isothermal lines limit species. 

 Nor will De Candolle's theory, that the limits are governed by the val- 

 ues of heat which are useful to a plant, assist the student ; for climatic 

 causes are not the only ones which limit vegetable species, or we should 

 then find the same species growing in every portion of any isothermal 

 belt of a continent, where the same conditions of heat and moisture 

 exist, which is not the case. Some species, apparently very local in 

 their habits and confined to a very limited area, are found many miles 

 farther north, with no intervening stations. For example : the Shizcea 

 pusilla, a little fern, was thought to be peculiar to New Jersey, where 

 it is confined to the pine-barren district, but it has lately been found 

 in Nova Scotia and Newfoundland, while no intervening stations have 

 as yet been reported. 



Here we have a plant, capable of propagating itself in New Jersey, 

 which was long thought to be its only home, reappearing several de- 

 grees farther north, where the climate is colder and otherwise different, 

 and yet unknown west of the Alleghany Mountains, where the climate 

 is very like that of New Jersey. Neither do we find many of the 

 plants of the western slope of the Alleghanies growing upon the east- 

 ern side. 



It is well known that if a piece of coniferous forest be cleared of its 

 timber, in Virginia or Pennsylvania, its site will soon be covered with 

 a growth of deciduous trees, but, if then left undisturbed, the conifer- 

 ous trees of the original growth will finally reassert their supremacy, 

 and in course of time the forest again becomes exclusively coniferous. 

 The black-walnut ( Juglans niger), which grows naturally from North 

 Carolina to the Great Lakes, and will grow with equal luxuriance on the 

 Pacific coast at latitude 45, bearing fruit which will germinate if 

 planted, has never yet been known by the writer to grow in Northern 

 Oregon if left to itself. I have examined the walnuts in the spring in 

 Oregon, which fell from the trees the previous fall they were inva- 

 riably rotten. Now, as one of the necessary conditions of plant-distri- 

 bution is the production of seed which will grow unaided by man upon 

 the soil which supports the parent, it follows that there is some other 

 cause than the requisite amount of heat that prevents the black-walnut 

 from becoming naturalized in Oregon. 



In the Smithsonian Report for 1858, page 246, is an article by Dr. J. 

 G. Cooper on the " Forests and Trees of North America," accompanied 

 with a map of North America north of Mexico. 



This map * is divided into provinces and regions, according to the 

 distribution of forest-trees, and the views herein maintained will be 

 more intelligible to the reader who will refer to it, and compare it with 

 a geological map of the same territory. 



* A better map forms the frontispiece to the Agricultural Report of the Patent-Office 

 Report for I860. 



