SOME ASPECTS OF GEOLOGIC TIME 265 



early Paleozoic deposits, and, of these, a considerable number of 

 genera and orders are of a remote antiquity. Thus, four out of 

 nine orders of insects have been found in the Carboniferous, 

 Crustacea in the Cambrian and pre-Cambrian, arachnida in the 

 Silurian. From these facts he infers that pre-Cambrian evolution 

 must have occupied a time vastly greater than that of which we 

 have a record. 



Though I am of opinion that this line of argument contains a 

 great amount of truth, I am bound to demur that all that can 

 definitely be asserted is an antecedent probability. If we assume, 

 as appears to be the case, that these invertebrate forms, at the 

 commencement of the period of the known fossiliferous strata, 

 had attained to correspondence with conditions that have 

 remained approximately constant during geologic time, we have 

 insufficient data on which to make definite assertions concerning 

 the time that preceded it. Let us put the matter more concretely. 

 It is very probable that all vertebrate life has developed from a 

 single type since the lower Cambrian. No phylum approaches 

 the vertebrates in the complexity of its ramifications. What 

 reason have we to assert that, when in process of active evolution, 

 each phylum found in the lower Cambrian could not have been 

 formed in an equal time ? And what reason have we to assert 

 that all these other phyla were not developed contemporaneously? 

 If we give to the argument its utmost value, we are unable to 

 assert that pre-Cambrian time has been greater than post- 

 Cambrian. The assertion that it is of equivalent length, which 

 is all the argument is worth, will help us very little. Such an 

 assertion is highly probable on other grounds. 1 A maximum 

 thickness of more than 100,000 feet of strata can definitely be 

 assigned to pre-Cambrian times, and the primitive Archaean 

 undoubtedly contains a large amount of metamorphosed sediment. 

 Such a conclusion is all we can obtain from this broad aspect of 

 biologic fact. Whatever time may be proved to have been 

 required to form Cambrian and post-Cambrian strata, to it must, 

 probably, be added at least an equal time for pre-Cambrian 

 strata. Whatever we may think concerning probabilities, it 

 would be rash dogmatism to assert more. 



The futility of dogmatism is also shown by the scarcity of 



1 Recent researches are showing the probability that pre-Cambrian time is, 

 probably, considerably greater than post-Cambrian. See address by Prof. A. P, 

 Coleman, British Association Report, Sheffield, igio. 



