(1) Insects and Crustaceans (Arthropoda).
(2) Cuttles, univalve and bivalve Shell-fishes (Mollusca).
(3) Worms (Annelida).
(4) Sea-urchins and Sea-stars (Echinodermata).
(5) Coral Animals, Sea-anemones, and Sea-jellies (Cœlenterata).
(6) Sponges, Foraminifera and Animalcules of simple organization (Protozoa).
There are, it is true, some animals allied to the mollusks and worms, which might be entitled to form separate groups, though of minor importance The position of the sponges is doubtful, and the great mass of Protozoa may admit of subdivision; but for our present purpose these six great groups or provinces of the Animal Kingdom may be held to include all the humbler forms of aquatic life, and they keep company with each other as far as the Early Cambrian. If, in accordance with the previous statements, we choose to divide the earth's history by the development of animal life rather than by rock formations, and to regard each period as presided over by dominant animal forms, we shall thus have an age of man, an age of mammals, an age of reptiles and birds, an age of amphibians and fishes, and an age of crustaceans and mollusks.
It is only within recent years that the researches more especially of Barrande, Hicks, Lapworth, Linarrson, Brögger, and others in Europe, and of Matthew, Ford and Walcott in America, have enlarged the known animals of the Lower Cambrian to nearly 200 species, and below this we know as yet very little of animal life. We may therefore take the Lower Cambrian, or "Olenellus Zone" as it has been called from one of its more important crustaceans,[3] as our starting-point for plunging into the depths below. In doing so, we may remark on the orderly and symmetrical nature of the chain of life, and on the strange fact that for so long ages animal life seems to have been confined to the waters, and to have undergone little development toward its higher forms. It is like a tree with a tall branchless stem bearing all its leaves and verdure at the top, or like some obscure tribe of men long living in isolation and unknown to fame, and then, under some hidden impulse or opportunity, becoming a great conquering and dominant nation. Or to compare it with higher things, it is like the Christian religion, for ages confined to a small and comparatively unimportant people, and developing slowly its faith and hopes, and then suddenly, under the personal influence of Christ and His apostles, spreading itself over the world, and in a few centuries becoming the ruling power in its greatest empire, surviving the fall of this and permeating all the great nations that sprang from its ruins. God's plans in nature, in history, and in grace seem to us very slow in their growth and maturity, but they are very sure.
LIFE IN THE EARLY CAMBRIAN