Now let us approach these facts from behind, watch their inception and growth, and see how unavoidable is the conclusion.
The life of any creature is primarily dependent on the regular renewal of its constituent particles. The process of living uses up the materials lived in. Living involves dying, and to postpone the dying the structure is continually supplied with fresh materials. This continuous supply of fresh materials we call nutrition. It is an increasingly elaborate process, with “many a slip ’twixt the cup and the lip”; and the main line of organic evolution is in development of these nutritive processes.
Conditions of the environment modify a creature, as in hide and hair; conditions of inter-animal competition modify him, as in horns and stings; conditions of reproduction modify him, developing an elaborate physical mechanism and a more elaborate scheme of decoration; but the most distinctive modification of a creature is that produced by its nutritive conditions. “Order Mammalia,” with all its towering superiority, is founded merely on a new way of feeding the baby. The food supply of the world is subject to fluctuating influences—climatic, geographic, and other; and as we watch the widening panorama of animal forms changing and growing up the ages, we see the whole procession to be moving always in one line—in pursuit of its dinner. We think of our dinners as a pleasing series of events, but we do not appreciate their awful importance.
The life of any creature absolutely depends on getting together a certain group of chemical constituents and keeping them reinforced. While those constituents, massed in certain proportions, are cunningly poured through a certain small orifice called a mouth, the creature lives. A procession of dinners passing a given point—that is the physical condition of life. We are the given point. If the procession goes another way—or stops awhile—“we” cease to live.
And since there is no law of nature calling on the proper constituents to arise, to detach themselves from their undesirable comrades, to form into rightly proportioned groups, appear at proper intervals and to enter the “given point,”—therefore the principal machinery of every living form is developed to discover, pursue, seize, and gather in these constituents. To obtain what we want from the air, gills and lungs are invented; that supply is so instantly imperative and so plentiful and easy of access, that an unconscious organic motion sucks it in. If food were as simple and common as oxygen we should be spared much exertion.
But food is anything but this. In its crude forms it is thinly scattered in the water, and small early beast-lets float around and grab it as they can. “You get food when it drops and you die when it stops—you helpless free agent of sorrow!”
Food in vegetable forms is also widespread and thin. The creatures that live on grass have had to develop the most cumbrous and involved of alimentary canals; huge barrels filled with many stomachs, supported by sturdy legs, as of tables, to hold the eating machine up, and carry it eternally about after its plentiful but highly diluted dinner. A concentrated vegetable food, like the fruit, brings out quite other qualities; as seen in all light swift arboreal animals, as the monkey; and between ground and tree rises the long neck of the giraffe—stretching, ever stretching, after his ascending dinner.
The humming-bird has slowly acquired a very special tongue to get his dinner, so has the butterfly; the tooth of the squirrel is necessitated by the stubborn nut; and the poor thirsting camel has his private portable food-and-water supply to meet the demands of life between far-scattered oases.
But when it appeared that food in predigested ready-to-eat packages was specially desirable; when the carnivorous habit was developed, then indeed we find a wild variety of adaptation to one’s dinner. Food in this form was not only widely scattered and difficult of access, but actively reluctant, sometimes even contentious. But means were found to encompass it. Was it small and hidden like the ant, yet numerous enough to pay for eating? Lo! the ant-eater’s slender snout and slenderer tongue pursue and capture it. Is it a fat grub, deep boring in the bark? The ingenious Javan monkey develops a special finger for his extradition. Does the insect fly waveringly from flower to flower? The bird flies more accurately and swiftly from insect to insect, and the hawk swoops still more efficiently from bird to bird.
Whatever form the dinner took, wherever the dinner went, there followed the fluent, ever-changing animal organism, producing tooth or claw, tongue or proboscis, seven stomachs or a private fish-pole—whatever was necessary to lure, catch, hold, inclose, and assimilate, this ever-receding and sometimes actively resisting, but always indispensable dinner. The evolution of animal organisms is conditioned mainly upon the food supply.