We should give at least brief attention to what may be called the primary, or necessary, conditions of life. We know that fishes cannot live very long out of water and that birds cannot live in water. These, however, are conditions which depend on the special ecological habits of these two particular kinds of animals. The necessity of a constant and sufficient supply of oxygen 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 must have food.

plasm, 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 of the really living substance in an animal's body contains water. This water, so necessary for the animal, may be derived from the general food, all of which contains water in greater or less quantity, or it may be taken apart from the other food by drinking or by absorption through the skin.

We know, too, that if the temperature is below a certain minimum point or above a certain maximum, these points varying for different animals, death takes the place of life. It is familiar knowledge that many animals can be frozen without being killed. Insects and other small animals may lie frozen through winter and resume active life again in the spring. An experimenter kept certain fishes frozen in blocks of ice at a temperature of -15° C. for some time and then gradually thawed them out unhurt. 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 icy powder. But a temperature of -20° C. killed the fish. According to L. J. Turner, the Alaska mud-fish (Dallia), was fed frozen to Esquimaux dogs. One of these thawing in the stomach of the animal made its escape alive. Frogs lived after being kept at a temperature of 28° C., centipedes, at

life.)

a temperature of -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 Amoeba show that these simplest animals contract and cease active motion at 35° C., but are not killed until a temperature of 40° to 50° C. is reached.

Variations in pressure of the atmosphere also constitute

conditions which may determine the existence of life. 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

FIG. 30.-Cephalopods. Lower figure, the devil-fish or octopus, Octopus punctatus. The upper figure represents the squid, Loligo pealii, swimming backward by driving a stream of water through the small tube slightly beneath the eyes. (From life, one-third natural size.)

its surface of fifteen pounds. That is, all animals living on the earth's surface or near it live under this pressure and under no other condition. The animals that live in water, however, sustain a much greater pressure, this pressure increasing with distance. Certain ocean fishes live habitually in great depths, at from two to nearly 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, their scales fall off because of the great expanse of the skin, and the stomach is thrust wrong side out. Indeed the body itself sometimes bursts. On

the other hand if an animal which lives normally on the surface of the earth is taken up a very high mountain or is carried up in a balloon to a great altitude where the pressure of the atmosphere is much less than at the earth's surface, serious consequences may ensue, and if too high an altitude is reached, death

occurs.

Some animals require certain organic salts or compounds of lime to form bones or shells, etc. These salts may be regarded as necessary articles of nutrition, though their function is not that of ordinary food. These are peculiar demands of special kinds of animals. There might also be included among primary life conditions such necessities as the light and heat of the sun, the action of gravitation, and other physical conditions

without which existence of life of any kind would be impossible on this earth.

Finally we may refer briefly to the "grand problem" of the origin of life itself. Any treatment of this question is bound to be wholly theoretical. We do not know a single positive thing about it. We have some negative evidence. That is, we have

no recorded instance-and men have searched diligently for examples of spontaneous generation. No protoplasm has been seen, or otherwise proved, to come into existence except through the agency of already existing protoplasm. All life comes from life. All those former beliefs of spontaneous appearance of bees from the carcasses of oxen, flies from

decaying flesh, hair worms from horse tail hairs in water troughs, and bacteria and infusoria in infusions of beef or hay have been shown on scientific investigation to be utterly without basis of fact.

But if protoplasm and life do not appear, are not being generated spontaneously in this earth epoch, may they not have been in earlier ages? Geologists and biologists attempt to explain most of the things that happened in earlier geologic ages by what they observe to be happening now. They would answer, on this basis, that what evidence we now have should lead us to believe that the generation of life has never occurred. But there must have been a beginning. Life has not always been. The accepted geological theory of the making of our earth precludes the existence of life on it until the globe was cool enough for organisms to exist. We know that there is a maximum of temperature beyond which protoplasm inevitably coagulates. When and where was this beginning of life? The biologist cannot admit spontaneous generation in the face of the scientific evidence he has. On the other hand he has difficulty in under standing how life could have originated in any other way than through some sort of transformation from inorganic matter.

As a matter of curiosity we may glance at a few of the