IX. The Evidences of Organic Evolution.
That the forms of life to-day found on the earth have come into existence by the evolution of the more complex forms from the simpler, and of these simpler forms from still simpler, through the ever-operating law of Selection, is a necessary conclusion from the following facts:
1. The existence in the animal world of all grades of structures, from the humblest possible protozoan, whose body consists of a single simple speck, to the most powerful and complex of mammals. There are estimated to be something like a million species of animals living on the earth to-day. There may be several times this number. These species are linked together by millions of varieties, and are so related to each other that they may be all gathered together into various genera; these genera may be grouped into families, the families into orders, and the orders into seven or eight great primary phyla. By taking existing species and adding to them the extinct species of the rocks, and placing them all according to their structural affinities, it is possible to arrange them in the form of a tree with the various phyla, orders, families, genera, and species, branching and rebranching from the main trunk. The existence of structures, so graduated as to render such an arrangement possible, is in itself suggestive of a common relationship and origin.
2. Evolution is suggested by the similarities and homologies of structure found throughout the animal kingdom. Some of these similarities and homologies have already been mentioned. They are everywhere—remoter and more fundamental, some of them, others closer and more detailed. To the untrained mind, which sees surfaces only, and not even surfaces well, the animal world is an interminable miscellany of forms. But to the biologist, who looks deeper and with immense acumen over the whole field of animal life, there are only seven or eight different types of structure in the entire animal world. These seven or eight types correspond with the primary classes, or phyla, into which animals are divided, viz., protozoa, sponges, celenterates, echinoderms, worms, mollusks, arthropods, and vertebrates. However widely the members of each of these great groups may differ among themselves in colour, size, habits of life, and the like, the members of each group all resemble each other fundamentally. Moles differ from monkeys, bats from men, and birds from crocodiles and toads. They differ enormously. But they are all vertebrates with red blood, double body cavities, backbones, two pairs of limbs, and five fingers on each limb. When they are looked at superficially, there is not much similarity between a water-strider and a butterfly or between a stag-beetle and a gnat. But they are all, in reality, built according to the same plan. Like all other insects, they have six legs, a sheath-like skeleton, and bodies characteristically divided into head, thorax, and abdomen. It is the same with all other great classes of beings. All worms resemble each other; and so do all mollusks, although they may differ in particulars as widely as nautiluses and clams. Echinoderms have a radiate structure, celenterates and sponges are vase-like in shape, and protozoa are one-celled. The differences in structure among the members of a group consist in different modifications of a fundamental type. Among the vertebrates the fore-limb may be an arm, a leg, a wing, a shovel, a flipper, or a fin. But in all cases it is the same organ—that is, the same implement modified to serve different ends. Take the mouth-parts of insects. In the grasshopper and cricket these parts are fitted for grinding; in the moths and butterflies they are fashioned into long tubes for sucking the sweets of flowers; in the mosquito they form an elaborate apparatus for drilling and drinking; and in the mayfly the mouth-parts, though present, are not used at all. In all of these animals these parts are essentially the same, although differing so much in their forms and purposes that the unscientific can scarcely be made to believe they are fundamentally alike. There is no fact more familiar to the biologist or more frequently met with in the fields of animal morphology than the fact that the same general type may be hammered into dozens, or hundreds, or even thousands, of different patterns by the incessant industry of its surroundings, and that the same organic part may be moulded into various implements serving totally different ends by the environmental vicissitudes of time and space. On the hypothesis that the members of each group of animals possessing common characteristics, whether the group be large or small, have sprung from a common ancestry, and that the differences in structure have arisen as a result of differences in environment, the similarities and homologies of structure existing among animals are perfectly intelligible. But on any other supposition they are inexplicable.
3. Evolution is suggested by the remarkable series of phenomena presented by embryology. There are at least four facts in the developmental history of every creature which can hardly be accounted for on any other supposition than that of organic evolution.
First, the fact that every animal, above the lowest, individually passes through an evolution between the beginning of its existence and its maturity. Terrestrial beings are not born, like Minerva, full-grown. They grow. They evolve. They commence close down to the very atoms. And from this lowly genesis they rise, through a series of marvellous changes, to that high state of perfection and greatness from which they descend to dissolution.
If we knew by actual observation as little concerning the evolution of individuals as we do of the evolution of species—if we had always been used to seeing animals, including ourselves, in full bloom—had never watched the tadpole, the pupa, and the babe pass through their wonderful metamorphoses on their way to maturity, it would probably be just as hard for many minds to believe that animals evolve individually to be what they are as it is for them to believe that species have grown to be what they are. In the case of individuals, however, the evolution takes place right before our eyes largely, while the evolution of species goes on so slowly and stretches back so far into the past that it can only be inferred.
Second, the fact that animals, no matter how much they may differ from each other at maturity, all begin existence at the same place. Every animal commences its organic existence as an egg—as a one-celled animal—as an organism identical in structure with the simplest protozoan. The ova of whales ‘are no larger than fern seeds.’ The eggs of the coral, the crab, the ape, and the man are so precisely alike that the highest powers of the microscope cannot distinguish between them.
Third, the fact that the members of the same great group of animals in their individual development pass through similar stages of evolution. The ‘worm’ stage in the development of most insects and the ‘fish’ stage of frogs are well known.
There are no more remarkable instances of individual evolution in the whole range of animal life. The fish, the reptile, the bird, the dog, and the human being—all vertebrates, in short—cannot for some time after their embryonic commencement be distinguished from each other. ‘The feet of lizards and mammals, the wings and feet of birds, and the hands and feet of men,’ says the illustrious Von Baer, as quoted by Darwin, ‘all arise from the same fundamental form’.[1a]