There is still another aspect of this question,--that of time. If neither the gradation of structural rank among adult animals, nor the gradation of growth in their embryological development gives us any evidence of a transition between types, does not the sequence of animals in their successive introduction upon the globe afford any proof of such a connection? In this relation, I must briefly allude to the succession of geological formations that compose the crust of our globe. The limits of this article will not allow me to enter at any length into the geological details connected with this question; but I will, in the most cursory manner, give a sketch of the great geological periods, as generally accepted now by geologists. The first of these periods has been called the Azoic or lifeless period, because it is the only one that contains no remains of organic life, and it is therefore supposed that at that early stage of the world's history the necessary conditions for the maintenance of animals and plants were not yet established. After this, every great geological period that follows has been found to be characterized by a special set of animals and plants, differing from all that follow and all that precede it, till we arrive at our own period, when Man, with the animals and plants that accompany him on earth, was introduced.
There is, then, an order of succession in time among animals; and if there has been any transition between types and classes, any growth of higher out of lower forms, it is here that we should look for the evidence of it. According to this view, we should expect to find in the first period in which organic remains are found at all only the lowest type, and of that type only the lowest class, and, indeed, if we push the theory to its logical consequences, only the lowest forms of the lowest class. What are now the facts? This continent affords admirable opportunities for the investigation of this succession, because, in consequence of its mode of formation, we have, in the State of New York, a direct, unbroken sequence of all the earliest geological deposits.
The ridge of low hills, called the Laurentian Hills, along the line of division between Canada and the States was the first American land lifted above the ocean. That land belongs to the Azoic period, and contains no trace of life. Along the base of that range of hills lie the deposits of the next great geological period, the Silurian; and the State of New York, geologically speaking, belongs almost entirely to this Silurian period, with its lowest Taconic division, and the Devonian period, the third in succession of these great epochs. I need hardly remind those of my readers who have travelled through New York, and have visited Niagara or Trenton, or, indeed, any of the localities where the broken edges of the strata expose the buried life within them, how numerous this early population of the earth must have been. No one who has held in his hand one of the crowded slabs of sand--or lime-stone, full of Crustacea, Shells, and Corals, from any of the old Silurian or Devonian beaches which follow each other from north to south across the State of New York, can suppose that the manifestation of life was less multitudinous then than now. Now, what does this fossil creation tell us? It says this: that, in the Silurian period, the first in which organic life is found at all, there were the three classes of Radiates, the three classes of Mollusks, two of the classes of Articulates, and one class of Vertebrates. In other words, at the dawn of life on earth, the plan of the animal creation with its four fundamental ideas was laid out,--Radiates, Mollusks, Articulates, and Vertebrates were present at that first representation of life upon our globe. If, then, all the primary types appeared simultaneously, one cannot have grown out of another,--they could not be at once contemporaries and descendants of each other.