THE GEOLOGIC FUNCTIONS OF LIFE. 663 



crustaceans, myriopods, spiders, and insects. The hard parts of their 

 bodies are mainly horny or chitinous forms of organic matter, and hence 

 their rehcs differ notably from the inorganic calcareous and silicious 

 remains of most of the preceding forms. The Arthropoda did not at 

 any time form a notable stratum of rock. Their geologic value lies 

 chiefly in what they teach of the progress of hfe and its relations, and 

 the aid they render in correlation and identification. In these respects 

 the group is a notable one. It was represented in the early fossihferous 

 strata by the^trilobites, one of the most interesting of all types of fossils. 

 These were probably the most highly developed organisms of their 

 times and give the clearest hints of the stage of psychological and socio- 

 logical development that had been reached when first the record of life 

 is opened to us. The record of the myriopods, spiders, and insects dates 

 from the middle Paleozoic, and gives the first clear hints of animal life 

 on the land. 



The contribution of the Vertebrata. — In the vertebrates the dynamic 

 or working organism may be said to reach its highest expression, unless 

 it be in the flying insects, and their inorganic residue becomes rela- 

 tively unimportant in rock formation. Although the greatest of all 

 animal types in most respects, it has never formed more than trivial 

 beds of rocks. There are occasional "bone beds," but they are 

 Ihin and limited in extent, and only partially formed of vertebrate 

 matter. The geological importance of the vertebrates lies in the higher 

 field of life evolution and in its mental accompaniment. Fishes 

 excepted, the vertebrates are mainly land types, and have for their chief 

 colleagues plants and insects. The other groups of animals are mainly, 

 though not wholly, marine. The vertebrates have little place in the 

 Paleozoic record, except near its close, but they dominate the Mesozoic 

 and Cenozoic eras, and are conspicuously the master type to day. 



III. THE ASSOCIATIONS AND ECOLOGICAL RELATIONS OF LIFE. 

 A. The Basis of Floras and Faunas. 

 Geologic interest is not confined to the kinds of plants and animals 

 that have lived and the contributions they have made to the deposits, 

 but embraces also their assemblage into floras and faunas, and the rela- 

 tions of these assemblages to the prevailing physiographic features. 

 These assemblages and relationships are among the most suggestive 

 factors of the earth's evolution, and are the most instructive for pur- 



