192 THE WORLD OF LIFE CHAP. 



equalled the whole series which we now know above it 

 Dr. Croll declared, that " whatever the present mean thick- 

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

 must be small in comparison with the mean thickness 

 of all the sedimentary 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." l This view was 

 supported by Sir Andrew Ramsay, 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." 2 



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. 



Passing through the long series of Lower Silurian strata, 

 (now separated as Ordovician) we have fuller developments 

 and more varied forms of the same classes found in the 

 Cambrian ; but in the Upper Silurian we meet with remains 

 of fishes, the first of the great series of the vertebrates to 

 appear upon the earth. They are of strange forms and 

 low type, mostly covered with a kind of plate-armour, and 

 apparently without any lower jaw. Hence they form a 

 separate class Agnatha ("without jaws"). They also 

 appear to have had no hard, bony skeleton, as the only parts 

 fossilised are the outer skin with its more or less armoured 

 covering. The illustration (Fig. 39) shows one of the simpler 



1 Origin of Species, 6th ed. p. a86< 2 Proc. Roy. Soc., 1874, p. 334. 



