stances 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.

3. 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 inorganic 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.

4. Primary conditions of animal life.-Certain primary conditions are necessary for the existence of all animals. We know that fishes can not live very long out of water, and that birds can not live in water. These, however, are special conditions which depend on the special structure and habits of these two particular kinds of backboned animals. But the necessity of a constant and sufficient supply of air is a necessity common to both; it is one of the primary conditions of their life. All animals must have air. Similarly both fishes and birds, and all other animals as well, must have food. This is another one of the primary conditions of animal life. That backboned animals must find somehow a supply of salts or compounds of lime to form into bones is a special condition peculiar to these animals.

5. Food. All the higher plants, those that are green (chlorophyll-bearing), can make their living substance out of inorganic matter alone-that is, use inorganic substances. as food. But animals can not do this. They must have already formed organic matter for food. This organic matter may be the living or dead tissues of plants, or the living or dead tissues of animals. For the life of animals it is necessary that other organisms live, or have lived. It is this need which primarily distinguishes an animal from a

plant. Animals can not exist without plants. The plants furnish all animals with food, either directly or indirectly. The amount of food and the kinds of food required by various kinds of animals are special conditions depending on the size, the degree of activity, the structural character of the body, etc., of the animal in question. Those which do the most need most. Those with warmest blood, greatest activity, and most rapid change of tissues are most dependent on abundance, regularity, and fitness of their food. As we well know, an animal can live for a longer or shorter time without food. Men have fasted for a month, or even two months. Among cold-blooded animals, like the reptiles, the general habit of food taking is that of an occasional gorging, succeeded by a long period of abstinence. Many of the lower animals can go without food for surprisingly long periods without loss of life. But the continued lack of food results inevitably in death. Any animal may be starved in time.

If water be held not to be included in the general conception of food, then special mention must be made of the necessity of water as one of the primary conditions of animal life. Protoplasm, the basis of life, is a fluid, although thick and viscous. To be fluid its components must be dissolved or suspended in water. In fact, all the truly living substance in an animal's body contains water. The water necessary for the animal may be derived from the other food, all of which contains water in greater or less quantity, or may be taken apart from the other food, by drinking or by absorption through the skin. Sheep are seldom seen to drink, for they find almost enough water in their green food. Fur seals never drink, for they absorb the water needed through pores in the skin.

6. Oxygen.-Animals must have air in order to live, but the essential element of the air which they need is its oxygen. For the metabolism of the body, for the chemical

changes which take place in the body of every living animal, a supply of oxygen is required. This oxygen is derived directly or indirectly from the air. The atmosphere of the earth is composed of 79.02 parts of nitrogen (including argon), .03 parts of carbonic acid, and 20.95 parts of oxygen. Thus all the animals which live on land are enveloped by a substance containing nearly 21 per cent of oxygen. But animals can live in an atmosphere containing much less oxygen. Certain mammals, experimented on, lived without difficulty in an atmosphere containing only 14 per cent of oxygen; when the oxygen was reduced to per cent serious disturbances were caused in the animal's condition, and death by suffocation ensued when 3 per cent of oxygen was left in the atmosphere. Animals which live in water get their oxygen, not from the water itself (water being composed of hydrogen and oxygen), but from air which is mechanically mixed with the water. Fishes breathe the air which is mixed with or dissolved in the water. This scanty supply therefore constitutes their atmosphere, for in water from which all air is excluded no animal can breathe. Whatever the habits of life of the animal, whether it lives on the land, in the ground, or in the water, it must have oxygen or die.

7. Temperature, pressure, and other conditions. Some physiologists include among the primary or essential general conditions of animal life such conditions as favorable temperature and favorable pressure. It is known from observation and experiment that animals die when a too low or a too high temperature prevails. The minimum or maximum of temperature between which limits an animal can live varies much among different kinds of animals. It is familiar knowledge that many kinds of animals can be frozen and yet not be killed. Insects and other small animals may lie frozen through a winter and resume active life again in the spring. An experimenter kept certain fish frozen in blocks of ice at a temperature of -15° C.

for some time and then gradually thawed them out unhurt. Only very hardy kinds adapted to the cold would, however, survive such treatment. There is no doubt that every part of the body, all of the living substance, of these fish was frozen, for specimens at this temperature could be broken and pounded up into fine ice powder. But a temperature of -20° C. killed the fish. Frogs lived after being kept at a temperature of -28° C., centipedes at -50° C., and certain snails endured a temperature of -120° C. without dying. At the other extreme, instances are known of animals living in water (hot springs or water gradually heated with the organisms in it) of a temperature as high as 50° C. Experiments with Amaba show that these simplest animals contract and cease active motion at 35° C., but are not killed until a temperature of 40° to 45° C. is reached.

The pressure or weight of the atmosphere on the surface of the earth is nearly fifteen pounds on each square inch. This pressure is exerted equally in all directions, so that an object on the earth's surface sustains a pressure on each square inch of its surface exposed to the air of fifteen pounds. Thus all animals living on the earth's surface or near it, live under this pressure, and know no other condition. For this reason they do not notice it. The animals that live in water, however, sustain a much greater pressure, this pressure increasing with the depth. Certain ocean fishes live habitually at great depths, as two to five miles, where the pressure is equivalent to that of many hundred atmospheres. If these fishes are brought to the surface their eyes bulge out fearfully, being pushed out through reduced expansion; their scales fall off because of the great expansion of the skin, and the stomach is pushed out from the mouth till it is wrong side out. Indeed, the bodies sometimes burst. Their bodies are accustomed to this great pressure, and when this outside pressure is suddenly removed the body may be bursted. Sometimes such a fish is raised from its proper level by a struggle with its