GEOLOG Y. 113 
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 sup- 
posed 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 Fucoide, 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 pre- 
ceded 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- 
