ORIGIN AND EVOLUTION OF LIFE 319
Canadian pre-Cambrian rocks, some of the simpler protozoa gave rise to the next higher stage of animal evolution and to the adaptive radia- tion on land and sea of the Invertebrata. We are compelled to as- sume that the physico-chemical actions, reactions and interactions were sustained and rendered step by step more complex as the single cells passed into groups of cells, and these into organisms with two chief cell layers (Coelenterata), and finally into organisms with three chief cell layers.
The metamorphosis of the pre-Cambrian rocks has for the most part concealed or destroyed all the life impressions which were un- doubtedly made in the various continental or oceanic basins of sedi- mentation. Indirect evidences of the long process of life evolution are found in the great accumulations of limestone, and in the deposits of iron and graphite^^ which, as we have already observed, constitute certain proofs of the existence at enormously remote periods of lime- stone-forming algffi, of iron-forming bacteria and of a variety of chloro- phyll-bearing plants. These evidences begin with the metamorphosed sedimentaries overlying the basal rocks of the primal earth's crust. The discovery by Walcott** of the highly specialized and differentiated in- vertebrates of the Middle Cambrian seas completely confirms the proph- ecy made by Charles Darwin in 1859*® as to the great duration of pre- Cambrian time.
By Middle Cambrian time the adaptive radiation of the Invertebrata to all the conditions of life — ^in continental waters, along the shore lines, and in the littoral and pelagic environment of the seas — was governed by mechanical and chemical principles fundamentally similar to those observed among the protozoa, but distributed through myriads of cells and highly complicated tissues and organs, instead of being differentiated within a single cell as in the ciliate protozoa. Among the principal functions thus evolved were, first, a more complicated action, reaction and interaction with the environment and within the organism; second, a more eflScient locomotion in the quest of food, in the capture of food and in the escape from enemies, giving rise in some cases to skeletal structures of various t3rpes; third, offensive and de- fensive armature and weapons, including chemical modes of offence and defence and methods of burrowing.'® There are also protective coverings for sessile animals.
We find swiftly moving types with the lines of modern submarines, whose mechanical means of propulsion resemble those of the most primitive darting fishes {e. g., Sagitta and other chaetognaths) . Other
17 Barrel], Joseph. See Pirsson, Louis V., and Schuchert, Charles, 1915, p. 547.
IS Waleott, Charles D., 1911, 1912.
1* Darwin, Charles, 1859, pp. 306, 307.
20 B. W. Miner.
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