than those of climate arise from external surroundings—from absence of means of defense, from character of food, of air, of water, and the presence of various enemies. These conditions vary in their importance with each group of animals, yet apparently the least of them may be able to limit the range of species. To limit the range is the first step toward extinction, for to cease to advance is to retreat. Adverse conditions may invade even the heart of its distribution, causing reduction of numbers, which, if long continued, must mean rarity and final extermination. Extinction comes to those species we call rare, and its advent must be unnoticed. Circumstances become unfavorable to the growth or reproduction of some animal. Its numbers are reduced—it is rare—it is gone.
The air in Indiana but a few years since was dark with the hordes of passenger pigeons at the time of their fall migrations. The advance of a tree-destroying, pigeon-shooting civilization has gone steadily on, and now who has seen a passenger pigeon? I have seen them, and I have a skin or two in my collection, but the bird I knew as filling the trees in my boyhood is now in the same region an ornithological curiosity.
A very slight change in the environment of any species may be a matter of the greatest moment as regards its increase or permanence. The dependence of the clover on the number of cats in a certain neighborhood is an illustration given us by Mr. Darwin. The clover depends on the bumble-bee for the fertilization of its pods. The nests of the bumble-bee are destroyed by the field mouse, which is thus an enemy of the clover. The balance is restored by the work of the cat, who captures the mouse and prevents its ravages on the nests of the bee. The old nursery jingle of the cow that tossed the dog that worried the cat that killed the rat is repeated throughout nature. With any change in any of the elements in this series the whole equilibrium of nature is interrupted. For this equilibrium is apparent only—a sort of armed neutrality, an established order of things which the superficial observer mistakes for real peace and permanence.
In some groups we find evidence of a progressive adaptation of individuals to circumstances—for example, to climate, ending in the formation of new species to accord with changed conditions of temperature. We may illustrate this by means of the arctic birches. In Norway, as in most northern regions with a moist climate, there are large forests of birches. In the valleys, where the summers are warm and reasonably long, the birches of different species grow to be considerable trees. Farther to the north, or higher up the mountains, the summer is too short for the growth of birch-trees, and their place is taken by birches which never pass beyond the size of small bushes. Still higher up there