Just as we can trace every individual animal to a microscopic germ in which all its parts were potentially present, so we can trace species, genera, and larger groups of animals to their commencement at different points of the earth's history, and can endeavour to follow the lines of creation or descent back to the first beings in which vital powers manifested themselves. All such beginnings must end in mystery, for as yet we do not know how either a germ or a perfect animal could originate from inanimate matter; but we may hope at least to make some approximation to the date of the origin of life and to a knowledge of the conditions under which it began to exist, confining ourselves for the present principally to the Animal Kingdom.
As preliminary to the consideration of this subject, we may shortly notice the grades of animals at present existing, and then the evidence which we have of their successive appearance in different periods of geological time, in order that we may eliminate all those of more recent origin, in so far as the knowledge at present available will permit, and restrict our consideration to forms which seem to have been the earliest. In attempting this, we may use for reference the table of geological periods and animal types presented in the diagram facing this chapter, which is based on one prepared by Prof. Charles A. White, of the United States Geological Survey, with modifications to adapt it to our present purpose. In this table the leading groups of animals are represented by lines stretching downward in the geological column of formations as far as they have yet been traced. Such a table, it must be observed, is always liable to the possibility of one or more of its lines being extended farther downward by new discoveries.
The broadest general division of the Animal Kingdom is into back-boned animals (Vertebrates) and those which have no back-bone or equivalent structure (Invertebrates).[1] The former includes, besides man himself, the familiar groups of Beasts, Birds, Reptiles, and Fishes. The latter consists of the great swarms of creatures included under the terms Insects, Crustaceans, Worms, Cuttle-fishes, Snails, Bivalve Mollusks, Star-fishes, Sea-urchins, Coral Animals, Sea-jellies. Sponges, and Animalcules. This mixed multitude of animals, mostly of low grade and aquatic. Includes a vast variety of forms, which, though comparatively little known to ordinary observers, are vastly numerous, of great interest to naturalists, and, as we shall find, greatly older in geological date than the higher animals.
[1] The twofold primary division now sometimes used, into Metazoa and Protozoa, seems more arbitrary and unequal, and therefore of less practical value.
It will be seen by a glance at the diagram that the higher vertebrates are of most recent origin, man himself coming in as one of the newest of all. Only the lower reptiles or batrachians and the fishes extend very far back in geological time. None of the other vertebrate groups reach, so far as yet known, farther back than the middle of the geological scale—probably in point of time very much less than this. Those of the invertebrates that breathe air reach no farther back than the fishes, possibly not so far. On the other hand, all the leading groups of marine invertebrates run without interruption back to the Lower Cambrian, and some of them still farther. Thus it would appear that for long ages before the introduction of land or air-breathing animals of any kind, the sea swarmed with animal life, which was almost as varied as that which now inhabits it. The reasons of this would seem to be that the better support given by the water makes less demands upon organs for mechanical strength, that the water preserves a more uniform temperature than the air, and that arrangements for respiration in water are less elaborate than those necessary in air. Hence the conditions of life are, so to speak, easier in water than in air, more especially for creatures of simple structure and low vital energy. Besides this, the waters occupy two-thirds of the surface of the earth, and in earlier periods probably covered a still greater area.
We are now in a position to understand that the Animal Kingdom had not one but many beginnings, its leading types arriving in succession throughout geological time. Thus the special beginning of any one line of life, or those of different lines, might form special subjects of inquiry; but our present object is to inquire as to the first or earliest introduction of life in our planet, and in what form or forms it appeared. We may, therefore, neglect all the vertebrate animals and the air-breathing invertebrates, and may restrict our inquiries to marine invertebrates.
In relation to these, six of the larger divisions or provinces of the Animal Kingdom may suffice to include all the lower inhabitants of the ocean, whether now or in some of the oldest fossiliferous rocks.[2]
[2] Some modern zoologists, having perhaps, like some of the old Greeks, lost the idea of the unity of nature, or at least that of one presiding divinity, prefer for the larger divisions of animals the term phylum or phylon, implying merely a stock, race or kind, without reference to a definite place in an ordered kosmos.
Looking more in detail at our diagram, we observe that the higher vertebrates nearest to man in structure extend back but a little way, or, with a few minor exceptions, only as far as the beginning of the Kainozoic or Tertiary Period, in the later part of which we still exist. Other air-breathing vertebrates, the birds and the true reptiles, extend considerably farther, to the beginning of the previous or Mesozoic Period. The amphibians, or frog-like reptiles, reach somewhat farther, and the fishes and the air-breathing arthropods farther still. On the other hand, our six great groups of marine invertebrates run back for a vast length of time, without any companions, to the lowest Palæozoic, and this applies to their higher types, the cuttles and their allies, and the crustaceans, as well as to the lower tribes. Turning now again to our table, we find that these creatures extend in unbroken lines back to the Lower Cambrian, the oldest beds in which we find any considerable number of organic remains, and leave all the other members of the Animal Kingdom far behind.
If now we endeavour to arrange the leading groups of these persistent invertebrates under a few general names, we may use the following, beginning with those highest in rank:—