I often think, when working over my plants, of what Linnæus once said of the unfolding of a blossom: "I saw God in His glory passing near me, and bowed my head in worship," The scientific aspect of the same thought has been put into words by Tennyson:—
"Flower in the crannied wall,
I pluck you out of the crannies,
I hold you here, root and all, in my hand,
Little flower,—but if I could understand
What you are, root and all, and all in all,
I should know what God and man is."
No deeper thought was ever uttered by poet. For in this world of plants, which with its magician chlorophyll conjuring with sunbeams is ceaselessly at work bringing life out of death,—in this quiet vegetable world we may find the elementary principles of all life in almost visible operation. It is one of these elementary principles—a very simple and broad one—that here concerns us.
One of the greatest contributions ever made to scientific knowledge is Herbert Spencer's profound and luminous exposition of Life as the continuous adjustment of inner relations to outer relations. The extreme simplicity of the subject in its earliest illustrations is such that the student at first hardly suspects the wealth of knowledge toward which it is pointing the way. The most fundamental characteristic of living things is their response to external stimuli. If you come upon a dog lying by the roadside and are in doubt whether he is alive or dead, you poke him with a stick; if you get no response you presently conclude that it is a dead dog. So if the tree fails to put forth leaves in response to the rising vernal temperature, it is an indication of death. Pour water on a drooping plant, and it shows its life by rearing its head. The growth of a plant is in its ultimate analysis a group of motions put forth in adjustment to a group of physical and chemical conditions in the soil and atmosphere. A fine illustration is the spiral distribution of leaves about the stem, at different angular intervals in different kinds of plants, but always so arranged as to ensure the most complete exposure of the chlorophyll to the sunbeams. Every feature of the plant is explicable on similar principles. It is the result of a continuous adjustment of relations within the plant to relations existing outside of it. It is important that we should form a clear conception of this, and a contrasted instance will help us. Take one of those storm-glasses in which the approach of atmospheric disturbance sets up a feathery crystallization that changes in shape and distribution as the state of the air outside changes. Here is something that simulates vegetable life, but there is a profound difference. In every one of these changes the liquid in the storm-glass is passive; it is changed and waits until it is changed again. But in the case of a tree, when the increased supply of solar radiance in spring causes those internal motions which result in the putting forth of leaves, it is quite another affair. Here the external change sets up an internal change which leads to a second internal change that anticipates a second external change. It is this active response that is the mark of life.
All life upon the globe, whether physical or psychical, represents the continuous adjustment of inner to outer relations. The degree of life is low or high, according as the correspondence between internal and external relations is simple or complex, limited or extensive, partial or complete, perfect or imperfect. The relations established within a plant answer only to the presence or absence of a certain quantity of light and heat, and to sundry chemical and physical relations in atmosphere and soil. In a polyp, besides general relations similar to these, certain more special relations are established in correspondence with the eternal existence of mechanical irritants; as when its tentacles contract on being touched. The increase of extension acquired by the correspondences as we ascend the animal scale may be seen by contrasting the polyp, which can simply distinguish between soluble and insoluble matter, or between opacity and translucence in its environment, with the keen-scented bloodhound and the far-sighted vulture. And the increase of complexity may be appreciated by comparing the motions respectively gone through by the polyp on the one hand, and by the dog and vulture on the other, while securing and disposing of their prey. The more specific and accurate, the more complex and extensive, is the response to environing relations, the higher and richer, we say, is the life.
IX
How the Evolution of Senses expands the World
The whole progression of life upon the globe, in so far as it has been achieved through natural selection, has consisted in the preservation and the propagation of those living creatures in whom the adjustment of inner relations to outer relations is most successful. This is only a more detailed and descriptive way of saying that natural selection is equivalent to survival of the fittest. The shapes of animals, as well as their capacities, have been evolved through almost infinitely slow increments of adjustment upon adjustment. In this way, for instance, has been evolved the vertebrate skeleton, through a process of which Spencer's wonderful analysis is as thrilling as a poem. Or consider the development of the special organs of sense. Among the most startling disclosures of embryology are those which relate to this subject. The most perfect organs of touch are the vibrissæ whiskers of the cat, which act as long levers in communicating impulses to the nerve-fibres that terminate in clusters about the dermal sacs in which they are inserted. These cat-whiskers are merely specialized forms of such hairs as those which cover the bodies of most mammals, and which remain in evanescent shape upon the human skin imbedded in minute sacs. Now in their origin the eye and ear are identical with vibrissæ. In the early stages of vertebrate life, while the differentiations of dermal tissue went mostly to the production of hairs or feathers or scales, sundry special differentiations went to the production of ears and eyes. Embryology shows that in mammals the bulb of the eye and the auditory chamber are extremely metamorphosed hair-sacs, the crystalline lens is a differentiated hair, and the aqueous and vitreous humours are liquefied dermal tissue! The implication of these wonderful facts is that sight and hearing were slowly differentiated from the sense of touch. One can seem to discern how in the history of the eye there was at first a concentration of pigment grains in a particular dermal sac, making that spot exceptionally sensitive to light; then came by slow degrees the heightened translucence, the convexity of surface, the refracting humours, and the multiplication of nerve-vesicles arranging themselves as retinal rods. And what was the result of all this for the creature in whom organs of vision were thus developed? There was an immense extension of the range, complexity, and definiteness of the adjustment of inner relations to outer relations; in other words, there was an immense increase of life. There came into existence, moreover, for those with eyes to see it, a mighty visible world that for sightless creatures had been virtually non-existent.