LIFE'S BORDERLAND
Although in some respects a deceptive resemblance may appear between the living and the nonliving, the distinction is definite. Living bodies, plant or animal, are made up of protoplasm, which, although mineral in substance, consists of a combination never found in the mineral kingdom. It gives to the body containing it the power of growth, and this growth is by additions from within. Minerals may increase in size, but only by additions from without. The prime characteristics of living organisms is that they reproduce their kind, given favorable conditions. Minerals never do so. A correlative of life and growth is death, but minerals never die. In the course of its career every animal or plant, in proportion to its need or the degree of complexity of its organs, develops within itself characteristic compounds, such as albumin, gluten, starch, cellulose, fat and other chemical results, not a trace of any of which is to be found in rocks or soil, or in the water or in the air. No distinction in nature is so absolute as that between the inorganic and the organic realms, the nonliving and living things, so far as our senses can perceive them.
When, however, we consider the two prime divisions of the living world—animal and vegetable—so diverse in their higher developments, we find them springing from the same base in a single cell of almost structureless protoplasm, and so alike in this simplest form as to be in some cases indistinguishable—mere drops of living matter whose functions are so limited that they present no discriminative characteristics. Indeed, marking a definite boundary between animals and plants may be difficult in cases much higher in the scale than these primitive globules of protoplasm.
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
No results of investigation in natural history have been more amazing than those that show the marvelous richness of the sea in plant and animal life—not merely at its warm margin, but far out in the centre of what the ancients used to call "the desert of waters"; not only at its surface, but in its profoundest depths, and under the polar ice as well as amid the tropics. Sea populations differ somewhat according to situation, those of the shallow shore lines, which are of the "littoral" fauna, differ largely from those living in the open sea and belonging to the "pelagic" fauna, and there are surface swimmers, and others confined to the abysses; but virtually every class and subdivision in the animal kingdom is represented in greater or less variety in the zoölogy of the ocean. The list stretches from the merest monads to the huge sharks and still bigger whales.
This multitude and diversity of animal life is possible in the sea because of an even greater plenitude of plants there, which furnish a never-failing food resource. Bacteria and blue-green algæ are at the base of this. Bacteria exist in all seas, as in all soils, and the fertility of nature above ground and under water depends on these microscopic organisms, whose numbers in the ocean are as incalculable as the grains of sand on its brink. In equal multitude are the diatoms, unicellular algæ with flinty cases, by which the waves are sometimes discolored over broad areas; and millions of other green plants, living alone, or in chains, minute in size, but each a chemical laboratory converting the salt water they absorb into meals for the animals that swallow them—animals in most cases almost as small and simple as the things they eat, and themselves destined to be sucked into the mouth of something a little bigger, to be in turn a tidbit for a third hungry mouth, and so on to the broiled mackerel for our own breakfast.