Part 3

Grenville Series on the Ottawa River (17 miles to an inch).

From Logan's Original Map of 1865.]

THE CHAIN OF LIFE TRACED BACKWARD IN GEOLOGICAL TIME

GEOLOGICAL CHRONOLOGY OF LIFE.

After Prof. C. A. White.

=Column Key=--Using First Letter ---------------------------------------- Invertebrates. Vertebrates. Plants. -------------- ------------- --------- Protozoa Ganoid Fishes Algæ Corals, etc. Telios Fishes Land Cryptogs Echinoids Batrachians Phænogams Worms Reptiles Mollusks Dinosaurs Arthropods Birds Insects Marsupials Land Snails Placentals Humans

Geological Invertebrates. Vertebrates. Plants. Systems or Periods. P C E W M A I L G T B R D B M P H A L P +------------------------------------------+ Kainozoic. | ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖| { Cretaceous ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖| { |-‖-‖-‖-‖-‖-‖-‖-‖--‖-‖-‖-‖-‖-‖-‖------‖-‖--| { Jurassic ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ | { |-‖-‖-‖-‖-‖-‖-‖-‖--‖---‖-‖-‖---‖------‖-‖--| { Triassic ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ | |-‖-‖-‖-‖-‖-‖-‖-‖--‖---‖-‖------------‖-‖--| Palæozoic. | ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ | { Permian ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ | { |-‖-‖-‖-‖-‖-‖-‖-‖--‖---‖--------------‖-‖--| { Carboniferous ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ | { |-‖-‖-‖-‖-‖-‖-‖-‖--‖------------------‖-‖--| { Devonian ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ | { |-‖-‖-‖-‖-‖-‖-‖----‖------------------‖-‖--| { Silurian ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ | { |-‖-‖-‖-‖-‖-‖-------------------------‖-‖--| { Ordovician ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ ‖ | { |-‖-‖-‖-‖-‖-‖-------------------------‖----| { Cambrian ...| ‖ ‖ ‖ ‖ ‖ ‖ ‖ | { |-‖-‖-‖-‖-‖-‖-------------------------‖----| { Etcheminian ...| ‖ ‖ ‖ ‖ ‖ ‖ | |-‖-‖-‖-‖-----------------------------‖----| Eozoic. | ‖ ‖ ‖ ‖ | { Huronian ...| ‖ · | { |-‖----------------------------------------| { Laurentian. | ‖ | { Grenvillian ...| ‖ | { |------------------------------------------| { Archæan ...| | | | +------------------------------------------+

Note.--It is not supposed that the Geological Periods were of equal lengths, as represented in the diagram.

ERRATA.

Where Cryptozoon prolificum occurs in the text, read Cryptozoon proliferum.

THE CHAIN OF LIFE TRACED BACKWARD IN GEOLOGICAL TIME

In infancy we have little conception of the perspective of time. To us the objects around us and even our seniors in age seem to have always been, and to have had no origin or childhood. It is only as we advance in knowledge and experience that we learn to recognise distinctions of age in beings older than ourselves. In thinking of this, it seems at first sight an anomaly, or at least contrary to analogy, that the oldest literature and philosophy deal so much with doctrines as to the origins of things. In this respect primitive men do not seem to have resembled children; and the fact that our own sacred records begin with answers to such questions, and that these appear in the oldest literary remains of so many ancient nations, and even in the folk-lore of barbarous tribes, might be used as an additional argument in favour of an early Divine revelation on such subjects, as a means of awakening primitive men to the comprehension of their own place in the universe.

However this may be, it is certain that modern science at first took a different stand.

The constancy of the motions of the heavenly bodies, our great time-keepers, and of the changes on the earth depending upon them, and the resolution of apparent perturbations into cycles of greater or less length, impressed astronomers and physicists with the permanence of the arrangements of the heavens and their eternal circling round without any change. In like manner, on the rise of geology, the succession of changes recorded in the earth seemed interminable, and Hutton could say that in the geological chronology he could see "no vestige of a beginning, no prospect of an end."

But the progress of investigation has changed all this, and has brought physical and natural science back to a position nearer to that of the old cosmogonies. Physical astronomy has shown that the constant emission of heat and light from the sun and other stars must have had a beginning, and is hurrying on toward an end, that the earth and its satellite the moon are receding from each other, and that even the spinning of our globe on its axis is diminishing in rapidity. In summing up these and other changes, Lord Kelvin says: "To hold the doctrine of the eternity of the universe would be to maintain a stupendous miracle, and one contrary to the fundamental laws of matter and force."

So, on our earth itself, we can now assign to their relative ages those great mountain chains which have been emblems of eternity. We can transfer ourselves in imagination back to a time when man and his companion animals of to-day did not exist, when our continents and seas had not assumed their present forms, and even when the earth was an incandescent mass with all its volatile materials suspended in its atmosphere. It is true that in all the changes which our earth has undergone the same properties of matter and the same natural laws have prevailed; but the interactions of these properties and laws have been tending to continuous changes in definite directions, and not infrequently to accumulations of tension leading to paroxysmal vicissitudes.

If all this is true of the earth itself, it is especially applicable to its living inhabitants. Successive dynasties of animals and plants have occupied the earth in the course of geological time; and as we go back in the record of the rocks, first man himself and, in succession, all the higher animals disappear, until at length in the oldest fossiliferous beds only a portion of the more humble inhabitants of the sea can be found. In the time of the formation of the oldest of these rocks, or perhaps somewhat earlier, must have been the first beginning of life on our planet.

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). 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.

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.

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:--

(1) Insects and Crustaceans (Arthropoda).

(2) Cuttles, univalve and bivalve Shell-fishes (Mollusca).

(3) Worms (Annelida).