“When we look at the plants and bushes clothing an entangled bank, we arc tempted to attribute their proportional numbers and kinds to what we call chance.  But how false a view is this!  Every one has heard that when an American forest is cut down, a very different vegetation springs up; but it has been observed that the trees now growing on the ancient Indian mounds, in the Southern United States, display the same beautiful diversity and proportion of kinds as in the surrounding virgin forests.  What a struggle between the several kinds of trees must here have gone on during long centuries, each annually scattering its seeds by the thousand; what war between insect and insect—­between insects, snails, and other animals, with birds and beasts of prey—­all striving to increase, and all feeding on each other or on the trees, or their seeds and seedlings, or on the other plants which first clothed the ground and thus checked the growth of the trees!  Throw up a handful of feathers, and all must fall to the ground according to definite laws; but how simple is this problem compared to the action and reaction of the innumerable plants and animals which have determined, in the course of centuries, the proportional numbers and kinds of trees now growing on the old Indian ruins!”—­(pp. 74, 75.)

For reasons obvious upon reflection, the competition is often, if not generally, most severe between nearly related species when they are in contact, so that one drives the other before it, as the Hanoverian the old English rat, the small Asiatic cockroach in Russia, its greater congener, etc.  And this, when duly considered, explains many curious results; such, for instance, as the considerable number of different genera of plants and animals which are generally found to inhabit any limited area.

“The truth of the principle that the greatest amount of life can be supported by great diversification of structure is seen under many natural circumstances.  In an extremely small area, especially if freely open to immigration, and where the contest between individual and individual must be severe, we always find great diversity in its inhabitants.  For instance, I found that a piece of turf, three feet by four in size, which had been exposed for many years to exactly the same conditions, supported twenty species of plants, and these belonged to eighteen genera, and to eight orders, which showed how much these plants differed from each other.  So it is with the plants and insects on small and uniform islets; and so in small ponds of fresh water.  Farmers find that they can raise most food by a rotation of plants belonging to the most different orders; Nature follows what may be called a simultaneous rotation.  Most of the animals and plants which live close round any small piece of ground could live on it (supposing it not to be in any way peculiar in its nature), and may be said to be striving to the utmost to live there; but it is seen that, where they come into the closest competition with each other, the advantages of diversification of structure, with the accompanying differences of habit and constitution, determine that the inhabitants, which thus jostle each other most closely, shall, as a general rule, belong to what we call different genera and orders.”—­(p. 114.)