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outside stimulus hold in all the organic world. We call animals and plants "organic" because they are made up of organs, cells, and tissues so grouped that like structures perform like functions. We could not use

a generic term like organic, were it not for the structural resemblances existing in each individual in great groups of organisms. All organisms have the impulse to reproduction. All are forced to make concession after concession to their surroundings and in such concessions progress in life consists. At last each organism or each alliance of organisms is dissolved by the process of death.

The unity in life is then not less a fact than the diversity. However great the emphasis we lay on individuality or diversity, the essential unity of life must not be forgotten. Whatever solution we may find to the problem of the origin of species, must also explain why species and individuals may be so much alike in all large details of structure. To know the origin of species we must also know why species admit of natural classification. Why is variety in life based on essential unity?

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From the fundamental unity of the species of to-day, we may infer the similar unity of species in past time. From the knowledge of variety in unity comes the likening of species of

animals or plants to the separated twigs of a tree, of which the trunk is more or less concealed. "We can only predicate and define species at all," says Dr. Elliott Coues, "from the mere circumstance of missing links. Our species are twigs of a tree separated from the parent stem. We name and arrange them arbitrarily, in default of a means of reconstructing the whole tree, in accordance with nature's ramifications." To continue

FIG. 14.-Heron flying. (After Marey.)

the figure, in our studies of the origin of the twigs of the tree, the existence of the trunk must not be forgotten. In the life of the earth variety in unity, unity in variety are nowhere separated.

Another equally striking simile is this: A species is an island, a genus, an archipelago, in a sea of death. The species is clearly definable only as its ancestors and cousins have disappeared, only in the degree that the stages in its development are unrepresented in our records. The genus is a group of species, an archipelago of islands, and there may be every conceivable degree of width or breadth of channel which seems to separate one island or group of islands from another.

CHAPTER III

LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION

There can be little doubt that the further science advances the more extensively and consistently will the phenomena of nature be represented by mathematical formula and symbols. But the man of science who, forgetting the limits of philosophical inquiry, slides from these formulæ and symbols into what is commonly understood by materialism, seems to me to place himself on a level with the mathematician who should mistake the x's and y's with which he works his problems for real entities, and with this further disadvantage as compared with the mathematician, that the blunders of the latter are of no practical consequence, while the errors of systematic materialism may paralyze the energies and destroy the beauty of a life.HUXLEY.

IN practice the distinction between a live thing and a lifeless one is usually of the simplest, but to define this distinction in terms so precise that the definition may be used as an invariable criterion is a problem of considerable difficulty. The sheep grazing in the field and the soil under its feet; the grass and flowers on the one hand, and the stones on the other hand, in the same pasture; there are no difficulties in the distinction here. Nor, indeed, even when we come to consider the simplest kinds of organisms, the tiny one-celled plants and animals that teem in stagnant waters of the wayside puddle. As we examine a drop of this water under the microscope we know without question what in it is alive and what in it is dead.

But let us attempt to put into words, into definite declaratory phrases, the characteristics of organisms and we find ourselves curiously impotent. When we come to study analytically organic nature and inorganic nature, things animate and

things inanimate, we find structures and behavior among inorganic things which cannot be readily distinguished in defining words from structures and behavior that are usually taken as characteristic of organisms. On the other hand we shall find in organic nature the very same chemical elements, and for the most part the same combinations of elements, that we find in the great mass of inorganic world substance. So that some biologists by a detailed and keen, if somewhat sophisticated, analysis of the alleged differences between animate and inanimate matter show that these differences are not absolute, and leave you with a stone in one hand and a grasshopper in the other logically unable to define the fundamental difference between the two, and yet morally certain of this absolute difference.

As a matter of fact there is one distinction between living matter and non-living matter which even the cleverest of the modern physicochemical school of biologists has as yet been unable to explain away. And that is the inevitable presence in living matter and the inevitable absence in non-living matter of certain highly complex chemical combinations of carbon, hydrogen, oxygen, nitrogen, and sulphur, called proteids or albuminous compounds.

The actual presence of these chemical substances in living matter is made manifest to us by the physicochemical behavior of these substances: that is, by our observation or recognition of their peculiar attributes. This behavior or these peculiar attributes or activities are those fascinating ones which we are accustomed to call the essential life processes. What these activities are we indicate in a not very precise way by the words organization, assimilation, growth, reproduction, motion, irritability, and adaptation. These essential life processes we have come by constant experience to associate always and only with a substance called protoplasm. Huxley long ago called protoplasm, therefore, the physical basis of life.

But protoplasm we have found to be a complex of substances or chemical compounds. Of these, a certain few are indispensable and fundamental, while others may be absent or present without affecting the particular capacities which make protoplasm the physical basis of life. This protoplasm too must be organized in a particular way in order that life may persist in the organism. It must appear in two conditions, and proto

plasmic stuff representing these two conditions must be disposed in certain definite relations. Protoplasm must occur as a cell or cells to be capable of performing the necessary activities of life. Hence we must consider at the very beginning of any discussion of life the two things, protoplasm and the cell.

The elements that compose protoplasm are the familiar ones, carbon, nitrogen, hydrogen, oxygen, sulphur, phosphorus, potassium, sodium, etc.; but these elements, or some of them, are found in protoplasmic cells in certain complex combinations which are not found elsewhere in nature, and which therefore actually and absolutely distinguish chemically living protoplasm from all lifeless matter. These particular combinations are certain albuminous compounds or proteids, composed of C, H, O, N, and S, and their complexity is extreme: the atoms in a single molecule often number more than a thousand. The molecules also are very large, which is probably the reason of their characteristic nondiffusibility through animal membranes or artificial parchment.

In addition to these characteristic albuminous compounds and various derivatives of them, protoplasm usually contains certain native albumins and certain other characteristic compounds known as carbohydrates and fats (which differ essentially from the albuminous substances in lacking nitrogen as a composing element). There are also various salts and gases and always water to be found in living substances. Water is absolutely necessary to the physical condition of half fluidity which gives to protoplasm its essential capacity for motion on itself. The commoner salts found in living substances are compounds of chlorine as well as the carbonates, sulphates, and phosphates of the alkalies and alkali earths, especially common salt (sodium chloride), potassium chloride, ammonium chloride, and the carbonates, sulphides, and sulphates of sodium, potassium, magnesium, ammonium, and calcium. The gases found in living matter are oxygen and carbon dioxide. These, when not in chemical combination, are almost always dissolved in water, although rarely they may be in the form of gas bubbles.

To sum up the relation of living matter to chemistry we may say that life is always associated with protoplasm, and that this protoplasm is made up of a few familiar inorganic elements, particularly those of lowest atomic weight; that it does not include any special so-called vital or life element, that is, any

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