A fundamental distinction between plants and animals as we now know them is the exclusive possession by plants of the green substance chlorophyll, by the presence of which their food is transformed under the influence of sunlight into vital energy in a manner essentially different from that by which animals assimilate their substance. Chlorophyll is a complex, nitrogenous, colloidal substance, produced by and always associated with, protoplasm, and related to the coloring matter of the blood of animals. It is restricted to plants, and usually resides only in definite portions of the cell; yet we have good reason for believing, as Prof. William F. Ganong tells us, that our present green plants were preceded in time by a colorless kind of the utmost simplicity, and without chlorophyll, which yet could make their own food from carbon dioxide and water by using the energy of chemical oxidation of soil-minerals in place of sunlight. "We have precisely such chemosynthetic organisms, a kind of soil bacteria, still living on the earth at this day; and they are doubtless the lineal descendants of the ancient forms, which probably lived in the mud of shallow seas that may be full of them yet." These ancient chemosynthetic organisms were neither animal nor plant, but both and between. They must have expanded, varied, evolved, thus originating a great many branches, most of which perished. Now, from this biochemical borderland of life, let us turn our attention to the living world as we know it to-day, or as preserved for us in the "record of the rocks," pausing only to fix well in our minds the main distinctions between animals and plants. Plants have no special organs for digestion or circulation, nor any nervous system. Most plants absorb inorganic food, such as water, carbonic acid gas, nitrate of ammonia, phosphates, silica, etc. No animal swallows any of these minerals as food. On the other hand, plants manufacture from such materials the food on which animals exist, by the production and storage in their tissues of starch, sugar, and nitrogenous substances. The two kingdoms supplement one another. They are mutually dependent, and probably originated simultaneously. CHAPTER II THE SEA A VAST AQUARIUM