292 THE ORIGIN OF SPECIES 



Consequently, if the theory be true, it is indisputable that before 

 the lowest Cambrian stratum was deposited long periods elapsed, 

 as long as, or probably far longer than, the whole interval from 

 the Cambrian age to the present day; and that during these vast 

 periods the world swarmed with living creatures. Here we en- 

 counter a formidable objection; for it seems doubtful whether 

 the earth, in a fit state for the habitation of living creatures, has 

 lasted long enough. Sir W. Thompson concludes that the consoli- 

 dation of the crust can hardly have occurred less than twenty or 

 more than four hundred million years ago, but probably not less 

 than ninety-eight or more than two hundred million years. These 

 very wide limits show how doubtful the data are; and other ele- 

 ments may have hereafter to be introduced into the problem. Mr. 

 CroU estimates that about sixty million years have elapsed since 

 the Cambrian period, but this, judging from the small amount of 

 organic change since the commencement of the Glacial epoch, 

 appears a very short time for the many and great mutations of 

 life, which have certainly occurred since the Cambrian formation ; 

 and the previous one hundred and forty million years can hardly 

 be considered as sufficient for the development 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 violent 

 changes in its physical conditions than those now occurring; and 

 such changes would have tended to induce changes at a corre- 

 sponding rate in the organisms which then existed. 



To the question why we do not find rich fossiliferous deposits 

 belonging to tJiese assumed earliest periods prior to the Cambrian 

 system, I can give no satisfactory answer. Several eminent geol- 

 ogists, 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 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 pecuHar species, beneath 

 the then known Silurian system; and now, still lower down in 

 the Lower Cambrian formation, Mr. Hicks has found South 

 Wales beds rich in trilobites, and containing various mollusks and 

 annelids. The presence of phosphatic nodules and bituminous mat- 

 ter, even in some of the lowest azotic rocks, probably indicates life 

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



