302 Causes and Course of Organic Evolution 



advent of a rich and highly evolved animal life at the beginning 

 of the Cambrian epoch, and by the entire absence, even of 

 traces, in the rocks of the archsean system. Thus graptolites, 

 brachiopods, lamellibranch gasteropod and cephalopod mol- 

 luscs, ostracods, and trilobites, that represent leading and 

 often highly evolved groups of invertebrates, are all found 

 to occur in the cambrian rocks. 



Even in the absence of direct fossil evidence, therefore, it 

 becomes a necessity of the case to assume that, throughout 

 a considerable stretch of archsean time, numerous and abundant 

 organic species and genera were evolving, but in part owing 

 to their soft and perishable nature, in part to the extensive 

 terrestrial changes proceeding, they have left no recognizable 

 trace of their existence. 



How far back into archsean times the evolutionary process 

 for organisms may have been carried, and how long may have 

 been the period during which organic forms were gradually 

 elaborating from the simplest bacteria and algae on to such 

 high types as the cephalopods, are questions which will be 

 considered in another place. But that the period was an 

 immensely extended one is conceded alike by geologists (p. 11) 

 and biologists, and judging from the thickness of mid-archsean 

 rock strata may conservatively be estimated at between 

 10,000,000 and 25,000,000 of years. We are compelled therefore 

 to accept it that during mid- and late-archsean times an abun- 

 dant flora and fauna originated; also that on the plant side 

 a degree of development had been reached that by the close 

 of archsean days probably represented the moss or bryophytic, 

 and even the simpler types of the fern or pteridophytic alli- 

 ance, while on the animal side all of the main groups of inver- 

 tebrate animals had been mapped out. The widespread 

 volcanic, metamorphic, and denudation changes proceeding, 

 combined with the often soft perishable substance of the earlier 

 plants and animals, would sufficiently explain their apparent 

 absence. 



As to the possible persistence of ancient species through 

 intervening periods, say from the cambrian up to the present 



