Page:Popular Science Monthly Volume 23.djvu/534

518 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 values 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 Shizæa 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 degrees 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 eastern 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 coniferous 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 invariably rotten. Now, as one of the necessary conditions of plant-distribution 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.