It is this inter-dependence of living things that makes the outcome of any direct interference with the natural order of things more or less problematical, and sometimes brings about results quite different from what were expected or intended.

The gamekeepers on the grouse moors of Scotland systematically killed off all birds of prey because they caught some of the grouse, but this is believed to have caused far more harm than good through permitting weak and sickly birds, that would otherwise have fallen a prey to hawks, to live and disseminate the grouse distemper.

The destruction of sheep by coyotes led the State of California to place a bounty on the heads of these animals, with the result that in eighteen months the State was called upon to pay out $187,485. As a result of the war on coyotes the animals on which they fed, notably the rabbits, increased so enormously that in turn a bounty was put on rabbits, the damage these animals caused the fruit-growers being greater than the losses among sheep-owners from the depredations of coyotes. And so, says Dr. Palmer, "In this remarkable case of legislation a large bounty was offered by a county in the interest of fruit-growers to counteract the effects of a State bounty expended mainly for the benefit of sheep-owners!"

Professor Shaler, in noting the sudden disappearance of such trees as the gums, magnolias, and tulip poplars from the Miocene flora of Europe has suggested that this may have been due to the attacks, for a series of years, of some insect enemy like the gipsy moth, and the theory is worth considering, although it must be looked upon as a possibility rather than a probability. Still, anyone familiar with the ravages of the gipsy moth in Ma.s.sachusetts, where the insect was introduced by accident, can readily imagine what _might_ have been the effect of some sudden increase in the numbers of such a pest on the forests of the past. Trees might resist the attacks of enemies and the destruction of their leaves for two or three years, but would be destroyed by a few additional seasons of defoliation.

Ordinarily the abnormal increase of any insect is promptly followed by an increase in the number of its enemies; the pest is killed off, the destroyers die of starvation and nature's balance is struck. But if by some accident, such as two or three consecutive seasons of wet, drought, or cold, the natural increase of the enemies was checked, the balance of nature would be temporarily destroyed and serious harm done. That such accidents may occur is familiar to us by the damage wrought in Florida and other Southern States by the unwonted severity of the winters of 1893, 1895, and 1899.

If any group of forest trees was destroyed in the manner suggested by Professor Shaler, the effects would be felt by various plants and animals. In the first place, the insects that fed on these trees would be forced to seek another source of food and would be brought into a silent struggle with forms already in possession, while the destruction of one set of plants would be to the advantage of those with which they came into compet.i.tion and to the disadvantage of vegetation that was protected by the shade. Finally, these changed conditions would react in various ways on the smaller birds and mammals, the general effect being, to use a well-worn simile, like that of casting a stone into a quiet pool and setting in motion ripples that sooner or later reach to every part of the margin.

It is scarcely necessary to warn the reader that for the most part this is purely conjectural, for from the nature of the case it is bound to be so. But it is one of the characteristics of educated man that he wishes to know the why and wherefore of everything, and is in a condition of mental unhappiness until he has at least formulated some theory which seems to harmonize with the visible facts. And from the few glimpses we get of the extinction of animals from natural causes we must formulate a theory to fit the continued extermination that has been taking place ever since living beings came into the world and were pitted against one another and against their surroundings in the silent and ceaseless struggle for existence.

THE END.