THE GEOLOGICAL EECORD 207 



well as the genus Lingnla, have continued to persist through 

 all the subsequent ages to the present time. Great masses 

 of rocks stratified and imstratified exist below the Cambrian, 

 but have mostly been metamorphosed by internal heat and pres- 

 sure, and contain no recognisable organic remains. 



Geologists have been greatly impressed by this sudden ap- 

 pearance of marine life in such varied forms and compara- 

 tively high organisation, and have concluded that the strati- 

 fied formations below the Cambrian must probably have 

 equalled the whole series which we now know above it. Dr. 

 Croll declared, that " whatever the present mean thickness of 

 all the sedimentary rocks of our globe may be, it must be 

 small in comparison with tlie mean thickness of all the sedi- 

 mentary rocks which have been formed " ; while Darwin says, 

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

 before the lowest Cambrian stratum was deposited long periods 

 elapsed, as long as, probably 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." ^ 

 This view was supported by Sir Andrew Eamsay, Director- 

 General of the Geological Survey, who possessed unrivalled 

 knowledge of the facts as to the geological record. He says, 

 speaking especially of the fossil fauna of the Cambrian age : 



" In this earliest known varied life we find no evidence of its 

 having lived near the beginning of the zoological series. In a 

 broad sense, compared with what must have gone before, both 

 biologically and physically, all the phenomena connected with this 

 old period seem, to my mind, to be of quite a recent description; 

 and the climates of seas and lands were of the very same kind 

 as those the world enjoys at the present day." - 



This consensus of opinion renders it highly probable that 

 the existing geological record only carries us back to some- 

 where about the middle of the whole period during which life 

 has existed upon the earth. 



1 Origin of Species, 6th ed. p. 286. 

 2proe. Roy. Soc., 1874, p. 834. 



