486 HISTORICAL GEOLOGY. 



amount of this gas is made equal to that of the oxygen in the present atmosphere, plants 

 will still thrive. How far this principle worked in early times is among the uncertainties. 

 The idea has been thrown out by T. Sterry Hunt that carbonic acid has been received 

 by the earth, from time to time, through the fall or near contact of meteorites, since carbon 

 exists sparingly in some of these bodies. But it has not found favor with astronomers. 



BIOLOGICAL PROGRESS. 



Display of the system of life in the Cambrian. — The system of life, as 

 exemplified by Lower Cambrian species, was essentially the existing system. 

 Seven of the grander divisions of animal life above the grade of Rhizopods 

 were represented : Sponges, Corals, Echinoderms, Worms, Brachiopods, Mol- 

 lusks, and Crustaceans. And under Mollusks there were species of Lamel- 

 libranchs, Pteropods or related forms, and Gastropods ; under Crustaceans, 

 Entomostracans of two sections, — the Ostracoids or Bivalve Crustaceans, 

 and Caridoids or shrimp-shaped species, — and Trilobites. It is true that 

 species are represented only by their hard parts — their shells or skeletons. 

 But the several subdivisions have species living in the existing world, so 

 that the nature of the life and the laws of structure and physiology of the 

 Cambrian species are, with feAv exceptions, all within man's range of study. 



The multiplicate structure a common, loiv-grade feature of the Cambrian 

 fauna. — The multiplicate structure exists among living species. But in the 

 early Paleozoic it was a prevalent feature under all the tribes that admitted 

 of it. The structure is a fundamental one in the Worms of all ages, the 

 body consisting of an indefinite number of body-segments; and, since succes- 

 sional lines of development led off from their precursors to Trilobites and 

 other Crustaceans, it is natural that Cambrian species of these classes should 

 be multiplicate in number of segments. The Protocarids (page 474) are 

 an example among the Crustaceans, as shown by the number of segments 

 in the abdomen ; the modern Apus is a representative of the Protocarid struc- 

 ture. The Ostracoids (Fig. 562) have their limbs and segments concealed by 

 the shell ; but there is reason to believe that these were multiplicate, and proto- 

 types of the modern Limnadia. The large Trilobites on pages 473, 476 exem- 

 plify the feature. Only the small Agnostus family (page 476) fails of it, and 

 these species probably fail because the form represents an embryonic condition. 



Other low-grade features of Loiver Cambrian species. — The Cystoids are 

 the lowest of Echinoderms, inferior to Crinoids. 



Brachiopods are (1) mostly the hingeless or inarticulate species ; (2) 

 small in size ; and (3), to a great extent, if the number of individuals of the 

 prevailing kinds is considered, species having a chitinous shell : and all 

 these characters are embryonic features. Further, as remarked by Schu- 

 chert, no species having spines or loops within the shell are yet known. The 

 special embryonic features of Kutorgina, Obolella, and Paterina have been, 

 well illustrated by Beecher; and, according to this author, the genus Kutor- 

 gina is probably the earliest representative of articulate Brachiopods. 



The chitinous shells of the Brachiopods, that make up so large a part of 

 the individuals, contain much calcium phosphate, as shown by the analyses 



