ends the green plant feeds on mineral matter, the animal on organic. Some plants have the power to form chlorophyl, the green coloring matter of leaves, which uses the energy of the sunlight to form starch out of the inorganic substances, — carbon dioxide and water. They are able also to form albuminoid matter out of inorganic substances. A very few animals which have a substance identical with or allied to chlorophyl have the same power, but in general animals are dependent for their food on the compounds put together in plants. Colorless plants, as fungi, possessing no chlorophyl, feed, like animals, on organic compounds. No living being is able to combine the simple elements carbon, oxygen, hydrogen, and nitrogen-into organic compounds. The food of plants is gaseous (carbon dioxide and ammonia) or liquid (water containing substances in solution), that of animals usually more or less solid, though solid substances must be changed to liquids before being capable of absorption into the tissues. The plant, then, absorbs these foods through its outer surface, while the animal takes its nourishment in larger or smaller masses, and digests it in a special cavity. A few exceptions, however, occur, since certain animals, as the tapeworm, have no digestive tract but absorb liquid food through the surface of the body. Plants are ordinarily fixed, their food is brought to them, and a large share of their work, the formation of organic compounds, is done by the energy of the sunlight; while animals are usually locomotive, must seek their food, and are unable to utilize the general forces of nature as the plant does. The plant is thus able to grow much more than the animal, as very little of the nourishment received is used to repair waste, while in most animals the time soon comes when waste and re pair are approximately equal. But in both all work done is paid for by waste of substance already formed. In combining carbon dioxide and water to form starch the plant sets oxygen free (6(CO2) + 5(H2O) = C6H10O5 +6(02)): in oxidizing starch or other food the animal uses oxygen and sets carbon dioxide free. The green plant in the sunlight, then, gives off oxygen and uses carbon dioxide, while plants, which have no chlorophyl, at all times, and all plants in the darkness, use oxygen and give off carbon dioxide, like an animal. Every plant begins life like an animal a consumer, not a producer not till the young shoot rises above the soil, and unfolds itself to the light of the sun, at the touch of whose mystic rays chlorophyl is developed, does real, constructive vegetation begin; then its mode of life is, in a sense, reversed; since more carbon is combined than liberated, and more oxygen set free than maintained. Most plants, and many animals, multiply by budding and division; on both we practice grafting; in both the cycle of life comes round again to the ovule or ovum. Do annuals flower but to die? Insects lay their eggs in their old age. Both animals and plants have sensibility. This is one of the fundamental physiological properties of protoplasm. But in plants the protoplasm is scattered and buried in rigid structures: feeling is, therefore, dull. In animals irritability is a highly developed property of certain organs, and so feeling, like electricity rammed into Leyden jars, goes off with a flash.65 Plants probably never possess consciousness or volition, as the higher animals do. The self-motion of animals and the rooted state of plants is a very general distinction; but it fails where we need it most. It is a characteristic of living things to move. The protoplasm of all organisms is unceasingly active.66 Besides this internal movement, myriads of plants, as well as animals, are locomotive. Rambling diatoms, writhing oscillaria, and the agile spores of cryptogams crowd our waters, their organs of motion (cilia and pseudopodia) being of the very same character as in microscopic animals; while sponges, corals, oysters, and barnacles are stationary. A contractile vesicle is not exclusively an animal property, for the several freshwater algæ, as Gonium, have it. The muscular contractions of the highest animals and the sensible motions of plants are both due to changes in the protoplasm in their cells. The ciliary movements of animals and of microscopic plants are precisely similar, and in neither case necessarily indicate consciousness or self-determining power. Plants, as well as animals, need a season of repose. Both have their epidemics. On both, narcotic and acrid poisons produce analogous results. Are some animals warm-blooded? In germination and flowering, plants evolve heat - the stamens of the arum, e.g., showing a rise of 20° F. In a sense, an oak has just as much heat as an elephant, only the miserly tree locks up the sunlight in solid carbon. At present, any boundary of the animal kingdom is arbitrary. "We cannot distinguish the vegetable from the animal kingdom by any complete and precise definition. Although ordinary observation of their usual representatives may discern little that is common to the two, yet there are many simple forms of life which hardly rise high enough in the scale of being to rank distinctively either as plant or animal; there are undoubted plants possessing faculties which are generally deemed characteristic of animals; and some plants of the highest grade share in these endowments.' "' 67 CHAPTER VI RELATION BETWEEN MINERALS, PLANTS, AND ANIMALS THERE are no independent members of creation: all things touch upon one another. The matter of the living world is identical with that of the inorganic. The plant, feeding on the minerals, carbon dioxide, water, and ammonia, builds them up into complex organic compounds, as starch, sugar, gum, cellulose, albumen, and gluten. When the plant is eaten by the animal, these substances are used for building up tissues, supplying energy, repairing waste, laid up in reserve as glycogen and fat, or oxidized in the tissues to produce heat. The albuminoids are essential for the formation of tissues, like muscle, nerve, cartilage; the ternary compounds help in repairing waste, while both produce heat. When oxidized, whether for work or warmth, these complex compounds break up into the simple compounds, — water, carbon dioxide, and (ultimately) ammonia, and as such are returned to earth and air from the animal. Both plant and animal end their life by going back to the mineral world: and thus the circle is complete from dust to dust. Plants compress the forces of inorganic nature into chemical compounds; animals liberate them. Plants produce; animals consume. The work of plants is synthesis, a building-up; the work of animals is analysis, or destruction. Without plants, animals would perish; without animals, plants had no need to be. CHAPTER VII* LIFE ALL forces are known by the phenomena which they cause. So long as the animal and plant were supposed to exist in opposition to ordinary physical forces or independently of them, a vital force or principle was postulated by which the work of the body was performed. It is now known that most, if not all, of the phenomena manifested by a living body are due to one or more of the ordinary physical forces, — heat, chemical affinity, electricity, etc. There is no work done which demands a vital force. The common modern view is that vitality is simply a collective name for the sum of the phenomena displayed by living beings. It is neither a force nor a thing at all, but is an abstraction, like goodness or sweetness; or, to use Huxley's expression, to speak of vitality is as if one should speak of the horologity of a clock, meaning its time-keeping properties. A third theory is still possible. The combination of elements into organic cells, the arrangement of these cells into tissues, the grouping of these tissues into organs, and the marshaling of these organs into plans of structure, call for some further shaping, controlling power to effect such wonderful coördination. Moreover, the manifestation of feeling and consciousness is a mystery which no physical hypothesis has cleared up. The simplest vital phenomenon has in it something over |