646 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



fossilization of marine animal types, therefore, first became possible in Cambrian 

 time simply because hard parts had then first become evolved. 



A principal and perhaps fatal objection to Brooks's idea is that there is no 

 apparent reason for the long postponement of the ' discovery of the sea-bottom.' 

 We can hardly doubt that, throughout the history of marine life, the shore zone 

 was as accessible to pelagic larvae, etc., as it is now and that the shore zone 

 afforded an advantageous habitat to marine organisms in pre-Cambrian time as 

 at the present. Professor Brooks agrees with most other authorities that the 

 time occupied in the evolution of the soft-bodied but highly diversified pelagic 

 species must have been enormous. It is scarcely conceivable that, in the time 

 taken to evolve such high types as cephalopods and trilobites, the shore zone 

 should not have been long successfully colonized. Skeletal and shell structures 

 should, therefore, have been developed several geological ages before the epoch 

 of high specific differentiation illustrated in the Cambrian. The conclusion seems 

 unavoidable that the sudden appearance of abundant fossils in certain Cambrian 

 beds is not due to a relatively late colonization of the shore zone. Everyone must 

 recognize the value of the shore zone as stimulating the evolutionary process, 

 but the Brooks hypothesis breaks down because it grants an inexplicable post- 

 ponement of the shore-line's influence. 



3. Suggested hypothesis. — A third hypothesis may be based on most of the 

 fundamental postulates of biology involved in Brooks's conception. Among these 

 may be specially recalled: («) the very slow evolution of higher animal types 

 from primordial, soft-bodied, simple types; (o) the supposition that the bulk of 

 marine animals and plants were, in pre-Cambrian time as now, pelagic and free- 

 swimming; (c) the further reasonable supposition that the pre-Cambrian sea was 

 tboroughly tenanted with animals. The point of departure of this third hypothe- 

 sis lies in the premise that, accepting these three postulates, it was impossible 

 during much of life's evolutionary period for animals to secrete limey structures 

 at all; for practical physiological purposes lime salts were non-existent in the 

 sea water for most of the pre-Cambrian life-period. 



So far as known to the writer, this hypothesis as a whole has not been 

 stated in geological or biological literature. Macallum has suggested that 

 calcium salts were but sparingly present in the ' earlier Archaean seas,' and notes 

 the possibility that pre-Cambrian organisms could therefore not have acquired 

 the ' lime-habit ' ; but he gives no explanation of the supposed small content of 

 lime in the sea-water.* Such explanation is the kernel of the hypothesis. 



The writer's sincere thanks are due to Mr. R. A. A. Johnston of the Cana- 

 dian Geological Survey for much help in discussing the basal chemical reactions. 



Precipitation of Lime Salts Through the Decomposition of Dead Organisms. 



It follows from the main biological postulates of the hypothesis that, in the 

 earliest sea, the higher animal types, including the active hunters and scavengers, 

 were not yet evolved. An important corollary is that the carcasses of countless 



* Transactions, Canadian Institute, Vol. 7, 1903, p. 536. 



