Yet, although there is no embryological transition of one type into another, the gradations of growth within the limits of the same type and the same class, already alluded to, are very striking throughout the Animal Kingdom. There are periods in the development of the germs of the higher members of all the types, when they transiently resemble in their general outline the lower representatives of the same type, just as we have seen that the higher orders of one class pass through stages of development in which they transiently resemble lower orders of the same class. This gradation of growth corresponds to the gradation of rank in adult animals, as established upon comparative complication of structure. For instance, according to their structural character, all naturalists have placed Fishes lowest in the scale of Vertebrates. Now all the higher Vertebrates have a Fish-like character at first, and pass successively through phases in which they vaguely resemble other lower forms of the same type before they assume their own characteristic form; and this is equally true of the other great divisions, so that the history of the individual is, in some sort, the history of its type. 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.