THE CAMBRIAN PERIOD 495 



life, for the life represented by the fossils could not have lived with- 

 out other life not represented. 



The Cambrian is the oldest system in which there is a reasonably 

 adequate record of life, and even here the record is far from com- 

 plete. The animal kingdom is fairly well represented among the 

 fossils, but plant remains are barely identifiable. 



The scantiness of plant fossils. The existence of plants in the 

 Cambrian period would perhaps be doubted were it not known 

 that all animals depend on them, directly or indirectly, for food. 

 It must be supposed, therefore, that plants abounded. The in- 

 adaptability of the lower plants to fossilization is doubtless the 

 chief explanation of their poor representation among the Cambrian 

 fossils. Reasons of a physical nature have been given previously 

 for thinking that the surface of the land was clothed with some 

 kind of vegetation. Yet there are no identifiable traces of land- 

 plants, and but obscure impressions of sea-plants. The lesson 

 taught is the extreme imperfection of the fossil record. 



The Animal Fossils 



Every great division of the animal kingdom, except the verte- 

 brate, had its representatives in Cambrian times. The Arthropoda 

 (p. 945) were represented by crustaceans; the Mollusca, by gastro- 

 pods, pteropods, and pelecypods; the Molluscoidea, by brachiopods; 

 the Vermes, by annelids; the Echinodermata, by cystoids; the 

 Coelenterata, by graptolites, medusae, and corals; the Porifera, by 

 sponges, and the Protozoa, by rhizopods. All the representatives 

 of these groups among the Cambrian fossils appear to be marine. 

 Of land animals there are no traces, but this does not prove that 

 land animals did not exist. Though no vertebrate remains have 

 yet been found, it would be rash to assume that none of them were 

 in existence. 



Arthropoda. Of the Arthropoda, crustaceans (represented now 

 by crayfish, crabs, etc.) only have been found in the Cambrian 

 strata. Their representatives were trilobites and entomostracans. 

 The trilobites were easily the most distinguished forms of life in 

 the Cambrian seas. They were not only the highest in organiza- 

 tion, but they were the most characteristic of the period. Their 



