84 GROUPS OF ALLIED SPECIES [Chap. X 



forms of life which already existed during the Cam- 

 brian period. It is, however, probable, as Sir William 

 Thompson insists, that the world at a very early period 

 was subjected to more rapid and violent changes in 

 its physical conditions than those now occurring ; and 

 such changes would have tended to induce 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 

 answer. Several eminent geologists, with Sir E. 

 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 conclusion. "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, abounding 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 con- 

 taining various molluscs and annelids. The presence of 

 phosphatic 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 

 Laurentian formation of Canada is generally admitted. 

 There are three great series of strata beneath the Silurian 

 system in Canada, in the lowest of which the Eozoon is 

 found. Sir W. Logan states that their " united thickness 

 " may possibly far surpass that of all the succeeding 

 " rocks, from the base of the palaeozoic series to the 



