Let us suppose our readers to have made themselves familiar with these organic remains, simply as characteristic and illustrative of this formation; they will easily find their way into other traces and remnants of ancient life in the Silurian epoch. How absurd must seem the development hypothesis to those who rightly ponder these old, old vestiges! It seems to us a very idle idea to suppose that a trilobite could develop itself into a bird, or a monkey, or by any series of happy accidents, could become a man;[33] yet such has been the theory of those who overlook what some writer on geology, whose name we forget, has expressed strongly in these words: “There is no fact which has been demonstrated more completely to the satisfaction of every man of real science, than that there is no known power in nature capable of creating a new species of animal, or of transmuting one species into another.” We close this chapter on the Silurian system in the eloquent words of Professor Sedgwick: “The elevation of the faunas of successive periods was not made by transmutation, but by creative additions, and it is by watching these additions that we get some insight into nature’s true historical progress. Judging by our evidence—and what else have we to judge by?—there was a time when Cephalopods were the highest type of animal life. _They_ were then the Primates of this world, and, corresponding to their office and position, some of them were of noble structure and gigantic size. But these creatures were degraded from their rank at the head of Nature, and _Fishes_ next took the lead; and they did not rise up in nature in some degenerate form, as if they were only the transmuted progeny of the Cephalopods, but they started into life in the very highest _ichthyic_ type ever reached.