PROGRESS OF LIFE. 597 



Believing in the unity and wisdom of the Divine plan, it is evident 

 that the discovery of the " missing links " in the succession of living 

 species, or, in other words, of the gradations between types, is one of 

 the grandest aims of geological science ; for only after a thorough 

 knowledge of all the facts will the system of life be completely under- 

 stood. 



5. The comprehensive character of many of the groups in past time. — 

 This principle runs through all geological history, and is, in fact, in- 

 volved in those already announced. 



The examples of comprehensive types illustrate the general truth 

 that the sub-kingdoms of life were present in early time, but in a 

 more condensed or comprehensive form than now — the grander 

 divisions having been defined, while the subordinate were often in com- 

 bination with one another, and became afterward differentiated. 



Among these comprehensive types, some are at or near a point of divergence of 

 lines in the system of progress, as the Crinoids, near the point of divergence of Comat- 

 ulids and Echinoids; some of the early Entomostracans, near that of modern Cyclo- 

 poids and Macrural Decapods; the earliest Tetradecapods, near that of Amphipods and 

 Isopods; the earliest Decapods, near that of Macrourans and Brachyurans; early 

 Neuropterous Insects, near that of true Neuropters and Orthopters ; the Ganoids, near 

 that of true Fishes and Reptiles, etc. 



Others, like the Brachiopods, Trilobites, and Cycads, are lines that appear to continue 

 undivided. There is no reason to suppose that a line from the Cycads led toward the 

 Palms, the structure of the plants being wholly against it; the Trilobites, before they 

 disappeared, were accompaniod by Tetradecapods; and there is nothing to support the 

 idea that from the Trilobites there were lines to the Tetradecapods. The Brachiopods 

 are the earliest known of 'Mollusks; but the line has no furcations afterward. The 

 Ascidian group, as it is the most fundamental comprehensive type under Mollusks 

 (and could not have been preserved in the rocks, since the body has no shell), may 

 have been the precursor of both the Brachiopods and ordinary Mollusks. 



6. The progress involved not only the expansion of types, but also 

 the culmination and decline of many, in the course of the history. — 

 (1.) The tribe of Crinoids began in the Primordial, culminated in the 

 Carboniferous age, and is now nearly extinct. 



(2.) Brachiopods have run a parallel course with the Crinoids ; the 

 families of the simple Lingula and Discina, with which the tribe 

 began, and a few other kinds of low grade now remain ; the genus 

 Leptcena, of great prominence in Silurian and Devonian time, had its 

 last species, as large as apple-seeds, in the Triassic. 



(3.) Trilobites began at the same time, in loose-jointed or flabby 

 species, with very large overgrown bodies and poor heads, passed their 

 climax in number, and apparently in grade, in the Silurian, and dis- 

 appeared, according to present evidence, at the close of the Paleozoic. 



(4.) Ganoid fishes began in the Upper Silurian, with vertebrated 

 tails ; rose out of this inferior condition, and passed their climax, as to 



