I have spoken of the first of these periods, the Azoic, as having been absolutely devoid of life, and I believe this statement to be strictly true; but I ought to add that there is a difference of opinion among geologists upon this point, many believing that the first surface of our globe may have been inhabited by living beings, but that all traces of their existence have been obliterated by the eruptions of melted materials, which not only altered the character of those earliest stratified rocks, but destroyed all the organic remains contained in them.  It will be my object to show, not only that the absence of the climatic and atmospheric conditions essential to organic life, as we understand it, must have rendered the previous existence of any living beings impossible, but also that the completeness of the Animal Kingdom in those deposits where we first find organic remains, its intelligible and coherent connections with the successive creations of all geological times and with the animals now living, afford the strongest internal evidence that we have indeed found in the lower Silurian formations, immediately following the Azoic, the beginning of life upon earth.  When a story seems to us complete and consistent from the beginning to the end, we shall not seek for a first chapter, even though the copy in which we have read it be so torn and defaced as to suggest the idea that some portion of it may have been lost.  The unity of the work, as a whole, is an incontestable proof that we possess it in its original integrity.  The validity of this argument will be recognized, perhaps, only by those naturalists to whom the Animal Kingdom has begun to appear as a connected whole.  For those who do not see order in Nature it can have no value.

[Illustration:  FOSSILS OF TRIASSIC VEGETATION.]

[Illustration:  BIRD OF THE JURASSIC PERIOD.(The Oldest Bird.)]

[Illustration:  SKELETON OF BIRD OF THE CRETACEOUS PERIOD.]

[Illustration:  SKELETON OF ANIMAL OF THE EOCENE PERIOD.]

For a table containing the geological periods in their succession, I would refer to any modern text-book of Geology, or to an article in the Atlantic Monthly for March, 1862, upon “Methods of Study in Natural History,” where they are given in connection with the order of introduction of animals upon earth.

Were these sets of rocks found always in the regular sequence in which I have enumerated them, their relative age would be easily determined, for their superposition would tell the whole story:  the lowest would, of course, be the oldest, and we might follow without difficulty the ascending series, till we reached the youngest and uppermost deposits.  But their succession has been broken up by frequent and violent alterations in the configuration of the globe.  Land and water have changed their level,—­islands have been transformed to continents,—­sea-bottoms have become dry land, and dry land has sunk to form sea-bottoms,—­Alps