ii8 THE ORIGIN AND EVOLUTION OF LIFE 



years, some of the simpler Protozoa gave rise to the next higher 



stage of animal evolution and to the adaptive radiation on 



land and sea of the Invertebrata. 



We are compelled to assume that the physicochemical actions, 



reactions, and interactions were sustained and became step by 



step more complex as the single-celled 



-,.. r /r. . \ 11-^ Phyla of Fossil 



hfe forms (Protozoa) evolved mto or- Invertebrata 



ganisms with groups of cells (Metazoa), Protozoa 



and these into organisms with two chief Porifera, 



cell-layers (Coelenterata), and later Coelenterata, 



. . 1 . r n Molluscoida, 



into organisms with three chief cell- Echinodermata 



layers. Annulata, 



The metamorphosis by heat and Mollu''s?a'^^' 



pressure of the pre-Cambrian rocks has 



for the most part concealed or destroyed all the life impressions 

 which were undoubtedly made in the various continental or 

 oceanic basins of sedimentation. Indirect evidences of the 

 long process of life evolution are found in the great accumula- 

 tions of limestone and in the deposits of iron and graphite^ 

 which, as we have already observed, are considered proofs of 

 the existence at enormously remote periods of limestone- 

 forming algae, of iron-forming bacteria, and of a variety of 

 chlorophyll-bearing plants. These evidences begin with the 

 metamorphosed sedimentaries overlying the basal rocks of the 

 crust of the primal earth. 



Pre-Cambrian and Cambrian Forms of Invertebrates 



The discovery by Walcotf- of a world of highly specialized 

 and diversified invertebrate life in the Middle Cambrian seas 

 completely confirms the prophecy made by Charles Darwin in 



1 Joseph Barrell. See Pirsson, Louis V., and Schuchert, Charles, 1915, p. 547. 

 - Walcott, Charles D., 1911, 1912. 



