bits has been that of the Norway rat over Eastern Europe and most of North America, and that of the English sparrow in the United States. Such weeds as the "jimson" (Jamestown) weed, the English charlock, the Canada thistle, and many other plants introduced into this country, have spread at an astonishing rate; and an insignificant water-plant (the Anacharis), on being introduced from this country into Great Britain, has there so increased as almost to block up rivers and canals, though here no trouble of the kind is noticed. Animals and plants, then, are kept from spreading so as to overrun the entire surface of the country in which they live, not by any lack of power to multiply so as to fill the space, but by the attacks of enemies of every description, as well as by heat, cold, moisture, drought, or famine; the last check being often produced by excessive crowding, either by members of their own or of other species. It has been remarked by those familiar with the woods of Northern Maine and the adjacent region, that deer are alternately plentiful and scarce. This seems to be almost certainly due to the fact, that, in the abundant years, wolves are attracted into the country from the extensive woods of Canada, and gradually increase in number till they destroy, or

drive out, very many of the deer. After this, upon their supply of food becoming scanty, the wolves, during successive years, either die out, or remove to more desirable hunting-grounds, thus leaving the few deer which have been spared to increase, comparatively unchecked until they again become abundant. But upon this the wolves are once more attracted back, and so the series of changes is repeated. In this instance, then, the number of deer is evidently, in great measure, dependent on the fewness of the wolves; or the scarcity of deer, upon the abundance of the wolves. In something the same way the grasshopper (locust) invasions in the South-western States have generally been checked by the rapid increase of insect parasites, which prey on the locusts to such an extent as to put a stop to their increase. So, again, in some portions of the wheat-raising region of the South-West, the chinch-bug has become so destructive to all kinds of grain, that the farmers are obliged to give up planting grain, and to raise crops (such as flax) which are not attacked by the bug.

The result seems to be, that, with the removal of its favorite food, the insect becomes less and less abundant till grain-raising once more becomes profitable. Whole regions of

country otherwise well adapted to sustain a given kind of animal may be rendered unfit to do so by the presence of some seemingly insignificant enemy. In portions of South Africa, according to Dr. Livingstone, the attacks of the tsetse fly make the country uninhabitable for horses, oxen, or dogs; though the bite of the fly is not specially injurious to other animals or

to man.

Paraguay, too, says Mr. Darwin, is unable to support wild horses, cattle, or dogs, from the presence of a fly, which is very abundant, and which deposits its eggs on the new-born young of the animals just mentioned. On the other hand, the existence of any of these flies (and, in fact, of most insects) depends very largely on the number of insect-eating birds in the country. The number of the latter, again, depends upon their greater or less liability to the attacks of birds of prey, of egg-eating birds, of snakes, and of many animals of the cat tribe. And these enemies of the insect-eating birds. are largely kept in check by human agency.

Starvation has been named as one of the enemies with which living beings have to deal, and it is indeed one of the most deadly. All are familiar enough with cases of its operation in the animal world, killing off, as it does, innu

merable individuals of all ranks, from man to the lowest. But among plants its action is not less widely fatal; and it is to this cause, in great part, that the "running-out" of old pastures, for instance, is due. The same cause, together with lack of sunlight, produces the barrenness of undershrubs, and even of smaller plants, which characterizes dense forests, such as those formed by the heavy growth of willows, sycamores, elms, and hackberries, that skirt the banks of so many Western rivers. In pine and other evergreen woods, the effect is still further heightened by the carpet of “ needles," or spills," which covers the ground so closely as effectually to prevent the growth of any but a few species of plants. To appreciate the kind and number of seeds of plants of all sorts, either lying dormant in the soil of pine-forests, or annually distributed upon it, one has only to observe the number of fire-weeds, raspberrybushes, and many other species, springing up quickly after the pine-woods have been cut down, and the ground burnt over.

And this first growth is, in turn, displaced by a colony of young oaks,1 interspersed with birches and alders; so that, in the course of

1 In some parts of New England sugar-maples and chestnuts also.

twenty-five years at most, the area at first cov ered with pine-forest will be found occupied by a growth of deciduous trees. These, on being cut away, will, in turn, be followed by pines and other evergreens.2

I have mentioned severe cold among the causes which destroy animals and plants, and there can be no doubt of its frequent and extensive agency in the work of destruction. Mr. Darwin has calculated, that from this cause, in the severe winter of 1854-55, four-fifths of the birds on his own grounds perished; and he goes on to say, "And this is a tremendous destruction, when we remember that ten per cent is an extraordinarily severe mortality from epidemics with man.” 3

No doubt many wild plants, too, are killed by extreme winters, just as we know is true of peach-trees, osage-oranges, and winter-wheat, among cultivated plants.

Enough has now been said, perhaps, to exemplify the cause and nature of the struggle for existence, as well as some of the living enemies and the hostile physical forces with which all organisms are obliged to contend. To the ac

1 Shedding the leaves in the autumn.

2 This statement is made from personal observations in Eastern New England nearly twenty years ago.

Origin of Species, p. 54.