84 GROUPS OP ALLIED SPECIES [Chap. X. 



varied forms of life which already existed during the 

 Cambrian period. It is, however, probable, as Sir Wil- 

 liam 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. Mur- 

 chison 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 dis- 

 puted this conclusion. We should not forget that only 

 a sriiall 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, 

 etill lower down in the Lower Cambrian formation, Mr. 

 Hicks has found in South Wales beds rich in trilobites, 

 and containing various molluscs and annelids. The 

 presence of phosphatie nodules and bituminous matter, 

 even in some of the lowest azoic rocks, probably indi- 

 cates life at these periods; and the existence of the 

 Eozoon in the Laurentian formation of Canada is gener- 

 ally admitted. There are three great series of strata be- 

 neath 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 palseo- 



