Star-fish. Associated with Brachiopods, etc., in great profusion, and in an admirable state of preservation, are the characteristic Trilobites (Fig. 144), an extinct order of Crustacea, to which the nearest approach at the present time is seen in minute Crustaceans like Cypris, favorite objects with the Microscopist, or like the larva of Limulus. In some genera of Trilobites, the different stages of their existence have been very well followed out, the fossils having been found perfect and in great profusion. The rocks furnish evidence of the existence of worms at this period, though, from the delicate nature of their bodies, their remains are few and obscure. Certain impressions or casts found in these rocks, known as Graptolites, are supposed to have been made by animals allied to the Sertularia of the present day, while the Niagara limestone consists almost entirely of Coral. The period characterized by the profusion of the remains of Brachiopods, Crinoids, and Trilobites is known as the Silurian, called after that of England and Wales, which derived its name from the ancient tribe of Silures, once inhabiting those parts. The plants of this period are Fucoidæ, or brown sea-weed. What conclusions can be drawn from the life of the Silurian period in favor of the theory of the higher forms having descended from the lower? We have seen that the animals of this period were aquatic. Now, animals living in the water are more simply and lowly organized than those living on land. An animal subjected to the ever-changing conditions of a land-existence needs a more complex organization to fulfill its requirements than one living in the comparatively unchanging sea. If the Development theory be true, the water-animals, then, should have preceded the land-animals, the water-plants the land-plants. Such has been the order of their appearance, according to the testimony of the rocks. In the chapter on Zoology we gave reasons for supposing that the Crinoids and Star-
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