90 THE ORIGIN OF SPECIES 



tations of life which have certainly occurred since the 

 Cambrian formation; and the previous 140 million years 

 can hardly be considered as sufficient for the develop- 

 ment of the varied forms of life which already existed 

 during the Cambrian period. It is, however, probable, 

 as Sir William Thompson insists, that the world at a 

 very early period was subjected to more rapid and vio- 

 lent changes in its physical conditions than those now 

 occurring; and such changes would have tended to in- 

 duce changes at a corresponding rate in the organisms 

 which then existed. 



To the question why we do not find rich fossiliferous 

 deposits belonging to these assumed earliest periods prior 

 to the Cambrian system, I can give no satisfactory an- 

 swer. Several eminent geologists, with Sir R. Murchison 

 at their head, were until recently convinced that we 

 beheld in the organic remains of the lowest Silurian 

 stratum the first dawn of life. Other highly competent 

 judges, as Lyell and E. Forbes, have disputed this con- 

 clusion. We should not forget that only a small portion 

 of the world is known with accuracy. Not very long 

 ago M. Barrande added another and lower stage, abound- 

 ing with new and peculiar species, beneath the then 

 known Silurian system; and now, still lower down in 

 the Lower Cambrian formation, Mr. Hicks has found 

 in South Wales beds rich in trilobites, and containing 

 various mollusks and annelids. The presence of phos- 

 phatic nodules and bituminous matter, even in some of 

 the lowest azoic rocks, probably indicates life at these 

 periods; and the existence of the Eozoon in the Lauren- 

 tian formation of Canada is generally admitted. There 

 are three great series of strata beneath the Silurian sys- 



