All this we learn from Zoologists, who have pursued their investigations without any reference whatever to the science of Geology. It is, therefore, not a little remarkable that we should discover this very order and gradation of animal life in the successive groups of Stratified Rocks. All the Remains hitherto discovered in the earliest Geological Formations belong to Invertebrate animals, while the Vertebrate, which appear for the first time in the latter part of the Silurian Period, are, from that age on, more and more fully developed down to the present day, and now constitute, if not the most numerous, at least the most important part of the animal creation. Moreover, it is to be observed that the Vertebrate animals do not all make their appearance at once, but come in successively according to the same scale of organic perfection,--the Fish appearing first, then the Reptiles, then the Birds, and lastly the Mammalia. Even among the Mammalia a well-defined order of progressive succession has been observed, which finally culminates in the appearance of Man, the last created and the most perfect of animals. [Illustration: TABLE OF GEOLOGICAL FORMATIONS, SHOWING THE FIRST APPEARANCE ON THE EARTH OF THE VARIOUS FORMS OF ANIMAL LIFE.] This remarkable succession of animal life in the history of the Earth’s Crust will be more readily understood by means of the annexed Table. The remains of Invertebrate animals have been traced as far back as the Lower Laurentian Rocks. The Vertebrate first become manifest in the Ludlow beds of the Upper Silurian; where they are represented by the bones of Fish, the lowest class belonging to the Province of Vertebrates. Next in order come the Reptiles: the oldest known Reptile having been found in the Coal Measures of Saarbrück between Strasburg and Treves. The skeletons of Birds are rare in the Stratified Rocks. It is supposed that their powers of flight have in all ages secured them, to great a extent, from being carried away by floods, like other land animals, and buried in the sedimentary deposits of rivers and estuaries. Nevertheless their presence in the ancient world is frequently attested by their footsteps, impressed originally on the sandy beach, and still preserved now that the soft sand has been converted into solid rock. Such traces have been discovered in great abundance on the New Red Sandstone of the Connecticut River in America; and afford the earliest evidence we possess in the records of Geology regarding the existence of the feathered tribe. This group of strata belongs to the lower Trias. In the higher beds of the same Formation we meet with the first relic of ancient Mammals. It was found near Stuttgardt, in 1847, and belongs to the more imperfect form of Mammalian life, the Non-Placental. Similar remains have been since discovered in the Upper Trias of Somersetshire. The Placental, or more perfect form of animal life in the same class, first appears in the