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). (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.