CŒLOMATA. VERTEBRATES. INVERTEBRATES. SURVEY OF THE ANIMAL KINGDOM BIRDS. Flying-Birds. Running-Birds. Placentals. Snakes. Lizards. REPTILES. Crocodiles. Tortoises. Corals. Ctenophores. Jellyfish. Sea-Anemones. STINGING-ANIMALS or CŒLENTERATES. Medusoids and Hydroids. SPONGES. Infusorians. Rhizopods. Gregarines. SIMPLEST ANIMALS. METAZOA. PROTO ZOA. PART IV THE EVOLUTION OF ANIMAL LIFE CHAPTER XVII THE EVIDENCES OF EVOLUTION 1. The Idea of Evolution—2. Arguments for Evolution: Physiological, Morphological, Historical—3. Origin of Life We observe animals in their native haunts, and study their growth, their maturity, their loves, their struggles, and their death; we collect, name, preserve, and classify them; we cut them to pieces, and know their organs, tissues, and cells; we go back upon their life and inquire into the secret working of their vital mechanism; we ransack the rocks for the remains of those animals which lived ages ago upon the earth; we watch how the chick is formed within the egg, and yet we are not satisfied. We seem to hear snatches of music which we cannot combine. We seek some unifying idea, some conception of the manner in which the world of life has become what it is. 1. The Idea of Evolution.--We do not dream now, as men dreamed once, that all has been as it is since all emerged from the mist of an unthinkable beginning; nor can we believe now, as men believed once, that all came into its present state of being by a flash of almighty volition. We still dream, indeed, of an unthinkable beginning, but we know that the past has been full of change; we still T believe in almighty volition, but rather as a continuous reality than as expressed in any event of the past. Thus Erasmus Darwin (1794), speaking of Hume, says "he concluded that the world itself might have been generated rather than created; that it might have been gradually produced from very small beginnings, increasing by the activity of its inherent principles, rather than by a sudden evolution of the whole by the Almighty fiat." In short, we have extended to the world around us our own characteristic perception of human history; we have concluded that in all things the present is the child of the past and the parent of the future. But while we dismiss the theory of permanence as demonstrably false, and the theory of successive cataclysms and re-creations as improbable,1 without feeling it necessary to discuss either the falsity or the improbability, we must state on what basis our conviction of continuous evolution rests. "La nature ne nous offre le spectacle d'aucune creation, c'est d'une continuation éternelle." "As in the development of a fugue," Samuel Butler says, "where, when the subject and counter-subject have been announced, there must thenceforth be nothing new, and yet all must be new, so throughout organic nature-which is a fugue developed to great length from a very simple subject— everything is linked on to and grows out of that which comes next to it in order-errors and omissions excepted." 2. Arguments for Evolution.--What then are the facts which have convinced naturalists that the plants and the animals of to-day are descended from others of a simpler sort, and the latter from yet simpler ancestors, and so on, back and back to those first forms in which all that succeeded were implied? I refer you to Darwin's Origin of Species (1859), where the arguments were marshalled in such a masterly fashion that they forced the conviction 1 I use the word in its literal sense-"not admitting of proof." It is not my duty nor my desire to discuss the poetical, or philosophical, or religious conceptions which lie behind the concrete cosmogonies of different ages and minds. To many modern theologians creation really means the institution of the order of nature, the possibility of natural evolution included. of the world. To the statements of the case by Spencer, Haeckel, Huxley, Romanes, and others, I have given references in the chapter on books. Darwin's arguments were derived (a) from the distribution of animals in space; (6) from their successive appearance in time, (c) from actual variations observed in domestication, cultivation, and in nature; (d) from facts of structure, e.g. homologous and rudimentary organs, (e) from embryology. I shall simply illustrate the different kinds of evidence, and that under three heads (a) physiological, (6) structural, (c) historical. (a) Physiological.—A study of the life of organisms shows that the ancient and even Linnæan dogma of the constancy or immutability of species was false. Organisms change under our eyes. They are not like cast-iron; they are plastic. One of the most striking cases in the Natural History Collection of the British Museum is that near the entrance, where on a tree are perched domesticated pigeons of many sorts-fantail, pouter, tumbler, and the likewhile in the centre is the ancestral rock-dove Columba livia, from which we know that all the rest have been derived. In other domesticated animals, even when we allow that some of them have had multiple origins, we find abundant proof of variability. But what occurs under man's supervision in the domestication of animals and in the cultivation of plants occurs also in the state of nature. Natural "varieties" which link species to species are very common, and the offspring of one brood differ from one another and from their parents. How many strange sports there are and grim reversions! and, as we shall afterwards see, modifications of individuals by force of external conditions are not uncommon. Those who say they see no variation now going on in nature should try a month's work at identifying species. I have known of an ancient man who dwelt in a small town; he did not believe in the reality of railways and to him the testimony of observers was as an idle tale; he was not daunted in his scepticism even when the railway was extended to his town, for he was aged, and remained at home, dying a professed unbeliever in that which he had |