Such are the pitcher-plants and sun-dews, and Venus-flytraps, which catch insects and use them for food nutrition. But there are many plants, the fungi, which are not green -that is, which do not possess chlorophyll, the substance on which seems to depend the power to make organic matter out of inorganic substances. These plants feed on organic matter as animals do. The cells of plants (in their young stages, at least) have a wall composed of a peculiar carbohydrate substance called cellulose, and this cellulose was for a long time believed not to occur in the body of animals. But now it is known that certain sea-squirts (Tunicata) possess cellulose. It is impossible to find any set of characteristics, or even any one characteristic, which is possessed only by plants or only by animals. But nearly all of the many-celled plants and animals may be easily distinguished by their general characteristics. The power of breaking up carbonic-acid gas into carbon and oxygen and assimilating the carbon thus obtained, the presence of chlorophyll, and the cell walls formed of cellulose, are characteristics constant in all typical plants. In addition, the fixed life of plants, and their general use of inorganic substances for food instead of organic, are characteristics readily observed and practically characteristic of manycelled plants. When the thousands of kinds of one-celled organisms are compared, however, it is often a matter of great difficulty or of real impossibility to say whether a given organism should be assigned to the plant kingdom or to the animal kingdom. In general the distinctive. characters of plants are grouped around the loss of the power of locomotion and related to or dependent upon it.

67. Living organic matter and inorganic matter.—It would seem to be an easy matter to distinguish an organism-that is, a living animal or plant-from an inorganic substance. It is easy to distinguish a dove or a sunflower from stone, and practically there never is any difficulty in making such distinctions. But when we try to define living organic matter,

and to describe those characteristics which are peculiar to it, which absolutely distinguish it from inorganic matter, we meet with some difficulties. At least many of the characteristics commonly ascribed to organisms, as peculiar to them, are not so. The possession of organs, or the composition of the body of distinct parts, each with a distinct function, but all working together, and depending on each other, is as true of a steam-engine as of a horse. That the work done by the steam-engine depends upon fuel is true; but so it is that the work done by the horse depends upon fuel, or food as we call it in the case of the animal. The oxidation or burning of this fuel in the engine is wholly comparable with the oxidation of the food, or the muscle and fat it is turned into, in the horse's body. The composition of the bodies of animals and plants of tiny structural units, the cells, is in many ways comparable with the composition of some rocks of tiny structural units, the crystals. But not to carry such rather quibbling comparisons too far, it may be said that organisms are distinguished from organic substances by the following characteristics: Organization; the power to make over inorganic substances into organic. matter, or the changing of organic matter of one kind, as plant matter, into another kind, as animal matter; motion, the power of spontaneous movement in response to stimuli; sensation, the power of being sensible of external stimuli; reproduction, the power of producing new beings like themselves; and adaptation, the power of responding to external conditions in a way useful to the organism. Through adaptation organisms continue to exist despite the changing of conditions. If the conditions surrounding an inorganic body change, even gradually, the inorganic body does not change to adapt itself to these conditions, but resists them until no longer able to do so, when it loses its identity or integrity.

CHAPTER VII

THE CROWD OF ANIMALS AND THE STRUGGLE FOR

EXISTENCE

68. The crowd of animals.—All animals feed upon living organisms, or on their dead bodies. Hence each animal throughout its life is busy with the destruction of other organisms, or with their removal after death. If those creatures upon which others feed are to hold their own, there must be enough born or developed to make good the drain upon their numbers. If the plants did not fill up their ranks and make good their losses, the animals that feed on them would perish. If the plant-eating animals were destroyed, the flesh-eating animals would in turn disappear. But, fortunately, there is a vast excess in the process of reproduction. More plants sprout than can find room to grow. More animals are born than can possibly survive. The process of increase among animals is correctly spoken of as multiplication. Each species tends to increase in geometric ratio, but as it multiplies its members it finds. the world already crowded with other species doing the same thing. A single pair of any species whatsoever, if not restrained by adverse conditions, would soon increase to such an extent as to fill the whole world with its progeny. An annual plant producing two seeds only would have 1,048,576 descendants at the end of twenty-one years, if each seed sprouted and matured. The ratio of increase is therefore a matter of minor importance. It is the ratio of net increase above loss which determines the fate of a species. Those species increase in numbers whose gain exceeds

the death rate, and those which "live beyond their means must sooner or later disappear. One of the most abundant of birds is the fulmar petrel, which lays but one egg yearly. It has but few enemies, and this low rate of increase suffices to cover the seas within its range with petrels.

It is difficult to realize the inordinate numbers in which each species would exist were it not for the checks produced by the presence of other animals. Certain Protozoa at their normal rate of increase, if none were devoured or destroyed, might fill the entire ocean in about a week. The congereel lays, it is said, 15,000,000 eggs. If each egg grew up to maturity and reproduced itself in the same way in less than ten years the sea would be solidly full of congereels. If the eggs of a common house-fly should develop, and each of its progeny should find the food and temperature it needed, with no loss and no destruction, the people of a city in which this might happen could not get away soon enough to escape suffocation from a plague of flies. Whenever any insect is able to develop a large percentage of the eggs laid, it becomes at once a plague. Thus originate plagues of grasshoppers, locusts, and caterpillars. But the crowd of life is such that no great danger exists. The scavenger destroys the decaying flesh where the fly would lay its eggs. Minute creatures, insects, bacteria, Protozoa are parasitic within the larva and kill it. Millions of flies perish for want of food. Millions more are destroyed by insectivorous birds, and millions are slain by parasites. The final result is that from year to year the number of flies does not increase. Linnæus once said that "three flies would devour a dead horse as quickly as a lion." Equally soon would it be devoured by three bacteria, for the decay of the horse is due to the decomposition of its flesh by these microscopic plants which feed upon it. "Even slow-breeding man," says Darwin, "has doubled in twenty-five years. At this rate in less than a thousand years there would literally not be standing room for his progeny. The elephant is reckoned the slow

est breeder of all known animals. It begins breeding when thirty years old and goes on breeding until ninety years old, bringing forth six young in the interval, and surviving till a hundred years old. If this be so, after about eight hundred years there would be 19,000,000 elephants alive, descended from the first pair." A few years more of the unchecked multiplication of the elephant and every foot of land on the earth would be covered by them.

Yet the number of elephants does not increase. In general, the numbers of every species of animal in the state of Nature remain about stationary. Under the influence of man most of them slowly diminish. There are about as many squirrels in the forest one year as another, about as many butterflies in the field, about as many frogs in the pond. Wolves, bears, deer, wild ducks, singing birds, fishes, tend to grow fewer and fewer in inhabited regions, because the losses from the hand of man are added to the losses in the state of Nature.

It has been shown that at the normal rate in increase of English sparrows, if none were to die save of old age, it would take but twenty years to give one sparrow to every square inch in the State of Indiana. Such an increase is actually impossible, for more than a hundred other species of similar birds are disputing the same territory with the power of increase at a similar rate. There can not be food and space for all. With such conditions a struggle is set up between sparrow and sparrow, between sparrow and other birds, and between sparrow and the conditions of life. Such a conflict is known as the struggle for existence.

69. The struggle for existence. The struggle for existence is threefold: (a) among individuals of one species, as sparrow and sparrow; (b) between individuals of different species, as sparrow with bluebird or robin; and (c) with the conditions of life, as the effort of the sparrow to keep warm in winter and to find water in summer. All three forms of this struggle are constantly operative and with