JR. A. Daly — Limeless Ocean of Pre- Cambrian Time. 95 



close of pre-Paleozoic time the pelagic fauna first discovered 

 the advantages of life alongshore and the special advantages 

 of life on the bottom of the shallow coast-waters. Owing to 

 the intense struggle for existence within the shore-zone, there 

 was, in early Cambrian time, a rapid acceleration of develoj)- 

 ment which tended towards the relatively sudden evolution of 

 hard calcareous and chitinous structures, which functioned, as 

 means of protection, of offence or of otherwise perfecting the 

 animals for successful combat. The 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 evolu- 

 tion 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 rela- 

 tively late colonization of the shore-zone. Everyone must 

 recognize the value of the shore-zone as stimulating the evolu- 

 tionary process, but the Brooks hypothesis breaks down because 

 it grants an inexplicable postponement of the shore-line's influ- 

 ence. 



3. A 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 : 

 (a) the very slow evolution of higher animal types from 

 primordial, soft-bodied, simple types ; (b) 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 thor- 

 oughly tenanted with animals. The point of departure of this 

 third hypothesis lies in the premise that, accepting these three 

 postulates, it was impossible during much of life's evolution- 

 ary 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. 



