Plain Facts for Old and Young | Page 6

discovery of this wonderful aid to human vision,--whole tribes of living organisms, each of which, though insignificant in size, possesses organs as perfect and as useful to it in its sphere as do animals of greater magnitude. Under a powerful magnifying glass, a drop of water from a stagnant pool is found to be peopled with curious animated forms; slime from a damp rock, or a speck of green scum from the surface of a pond, presents a museum of living wonders. Through this instrument the student of nature learns that life in its lowest form is represented by a mere atom of living matter, an insignificant speck of trembling jelly, transparent and structureless, having no organs of locomotion, yet able to move in any direction; no nerves or organs of sense, yet possessing a high degree of sensibility; no mouth, teeth, nor organs of digestion, yet capable of taking food, growing, developing, producing other individuals like itself, becoming aged, infirm, and dying,--such is the life history of a living creature at the lower extreme of the scale of animated being. As we rise higher in the scale, we find similar little atoms of life associated together in a single individual, each doing its proper share of the work necessary to maintain the life of the individual as a whole, yet retaining at the same time its own individual life.
As we ascend to still higher forms, we find this association of minute living creatures resulting in the production of forms of increasing complicity. As the structure of the individual becomes more complex and its functions more varied, the greater is the number of separate, yet associated, organisms to do the work.
In man, at the very summit of the scale of animate existence, we find the most delicate and wonderfully intricate living mechanism of all. In him, as in lower, intermediate forms of life, the life of the individual is but a summary of the lives of all numberless minute organisms of which his body is composed. The individual life is but the aggregate life of all the millions of distinct individuals which are associated together in the human organism.
Animals and Vegetables.--The first classification of living creatures separates them into two great kingdoms, animals and vegetables. Although it is very easy to define the general characteristics of each of these classes, it is impossible to fix upon any single peculiarity which will be applicable in every case. Most vegetable organisms remain stationary, while some possess organs of locomotion, and swim about in the water in a manner much resembling the movements of certain animals. Most vegetables obtain their nutriment from the earth and the air, while animals subsist on living matter. A few plants seem to take organic matter for food, some even catching and killing small insects.
It is found impossible to draw the precise line between animals and vegetables, for the reason just mentioned. The two kingdoms blend so intimately that in some cases it is impossible to tell whether a certain microscopic speck of life is an animal or a vegetable. But since these doubtful creatures are usually so minute that several millions of them can exist in a single drop of water, it is usually of no practical importance whether they are animal or vegetable, or sometimes one and sometimes the other, as they have been supposed to be by some biologists.
All living creatures are organized beings. Most possess a structure and an organism more or less complicated; but some of the lowest forms are merely little masses of a transparent, homogeneous jelly, known as protoplasm. Some of the smallest of these are so minute that one hundred millions of them could occupy the space of a cube one-thousandth of an inch on each side; yet each one runs its course of life as regularly as man himself, performing its proper functions even more perfectly, perhaps.
Life Force.--To every thinking mind the question often recurs, What makes the fragrant flower so different from the dead soil from which it grows? the trilling bird, so vastly superior to the inert atmosphere in which it flies? What subtle power paints the rose, and tunes the merry songster's voice? To explain this mystery, philosophers of olden time supposed the existence of a certain peculiar force which is called life, or vital force, or vitality. This supposition does nothing more than furnish a name for a thing unknown, and the very existence of which may fairly be doubted. In fact, any attempt to find a place for such a force, to understand its origin, or harmonize its existence with that of other well-known forces, is unsuccessful; and the theory of a peculiar vital force, a presiding entity present in every living thing, vanishes into thin air to give place to the more rational