The Palæozoic age of geology is thus emphatically an age of invertebrates of the sea. In this period they were dominant in the waters, and until toward its close almost without rivals. We shall find, however, that in the Upper Silurian, fishes made their appearance, and in the Carboniferous amphibian reptiles, and that, before the close of the Palæozoic, vertebrate life in these forms had become predominant. We shall also see that just as the leading groups of Mollusks and Crustaceans seem to have had no ancestors, so it is with the groups of Vertebrates which take their places. It is also interesting to observe that already in the Palæozoic all the types of invertebrate marine life were as fully represented as at present, and that this swarming marine life breaks upon us in successive waves as we proceed upward from the Cambrian. Thus the progress of life is not gradual, but intermittent, and consists in the sudden and rapid influx of new forms destined to increase and multiply in the place of those which are becoming effete and ready to vanish away or to sink to a lower place. Farther, since the great waves of aquatic life roll in with each great subsidence of the land, a fact which coincides with their appearance in the limestones of the successive periods, it follows that it is not struggle for existence, but expansion under favourable circumstances and the opening up of new fields of migration that is favourable to the introduction of new species. The testimony of palæontology on this point, which I have elsewhere adduced at length,[23] in my judgment altogether subverts the prevalent theory of “survival of the fittest,” and shows that the struggle for existence, so far from being a cause of development and improvement, has led only to decay and extinction, whereas the advent of new and favourable conditions, and the removal of severe competition, are the circumstances favourable to introduction of new and advanced species. This testimony of the invertebrates of the sea we shall find is confirmed by other groups of living beings, to be noticed in the sequel.[24] * * * * * NOTE.—The term “Siluro-Cambrian,” as used in this and the next chapter, is synonymous with “Ordovician” of Lapworth, which is now coming into somewhat general use. [Illustration: CORDAITES, OF THE GROUP OF DORY-CORDAITES. BRANCH RESTORED.—After Grand’ Eury.] CHAPTER IV. THE ORIGIN OF PLANT LIFE ON THE LAND.