GROWTH OF GEOLOGY. 247 



Cambrian and those now forming, we are forced to 

 make a large addition in order to account for the 

 evolution of the rich Cambrian fauna. 



Under the Cambrian beds there is evidence of some 

 80,000 feet of stratified rock, in which there are no 

 remains of organisms, but during which it seems al- 

 most necessary to assume that the chief types of back- 

 boneless animals and simple plants had their origin. 

 The absence of fossils is most plausibly interpreted 

 as mainly due to the absence of hard or preservable 

 parts in the primitive forms ; and even the modest es- 

 timate of twenty-six millions of years as the period, 

 since the earth became fit to be a home of life, leaves 

 a considerable number of millions for this pre- 

 Cambrian period during which the unicellular crea- 

 tures may have given origin to multicellular bodies, 

 taking the form of polyps and worms, even of trilo- 

 bites and molluscs. The suggestion has often been 

 made that in early times, among simple creatures, 

 the rate of progress may have been much more rapid 

 than among the higher forms whose stages of evolu- 

 tion are recorded in the rocks. But this is mere 

 opinion. 



At the beginning of the nineteenth century there 

 was an irrelevant belief that the habitable earth was 

 some 6,000 years old. But the work of James Hut- 

 ton alone was enough to convince the unprejudiced 

 that the antiquity of the earth must be inconceivably 

 great. The tendency of progressive geologists to 

 draw without stint upon the bank of time, had to face 

 a wholesome reminder from the physicists that their 

 credit was not unlimited. The limitations imposed 

 by the physicists have been vigorously rebelled 

 against, and criticism has tended to show that they 

 were too narrow and not altogether warrantable. The 



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