Page:Popular Science Monthly Volume 84.djvu/556

552 Michigan, southward in the eastern part of its range to Delaware, and in the west to southeastern Indiana and extreme southern Illinois, while it extends along the southern Appalachians to north central Georgia, central Alabama and Mississippi, and central Tennessee. This distribution is most easily remembered if we observe that it takes the general form of an ellipse, which is about twice as long as broad, with the southern end in central Mississippi, Alabama and Georgia, the northern end in Maine, New Hampshire and Vermont, while the greatest cross diameter of the ellipse extends from southeastern Michigan to Delaware (Fig. 1).

Although this is the area which now includes all chestnut growing naturally or "wild" in the United States, it does not necessarily represent the territory it has always occupied in the past. For geological evidence, as well as our own observational powers, show us that in both plant and animal worlds the confines of a species are constantly varying—now expanding, now contracting. This condition is evidently due to a great variety of factors, but at the very groundwork of them all lie the fundamental principles of the struggle for existence and the survival of the fittest. In modern times the modifying action of man on this perpetual contraction or expansion of a species has been by no means slight, and with the ever-increasing facilities of commerce, his influence is becoming more and more marked. To cite an example from the plant world, some of the most obnoxious weeds that grow about us to-day, and are the bane of the farmer, are intruders from foreign countries, their seeds having been brought in with various imported materials. Having thus arrived here, many of them find congenial soil and make their home among us, thereby considerably widening the range of distribution of their particular species. On the other hand, by the unwitting introduction of various fungous or insect parasites, man may be instrumental in the contraction or even the extinction of some of our plant or animal species.

Such examples as the bison, or the North American Indian, demonstrate how rapidly the distribution of a species or race may change, even within the memory of man.

Geological data, as furnished us in the form of fossils, are often illuminating as to the former distribution of our plant and animal species. For example, the giant big-tree and redwood, of California, quite