LIFE ON THE EARTH. 69 



natural system as that of the Lingula flags below, 

 and of the Caradoc beds above. Thus in tracing 

 backward the series of Siluro-Cambrian life it ap- 

 pears to diminish, and disappear in a few forms, if 

 not in a single form, before we reach the base of the 

 strata. Thus we seem to have, as nearly as can be 

 had, the evidence of the beginning of the earliest 

 traceable life after the commencement of the earliest 

 non-metamorphic strata. 



3. In tracing downwards the various classes of 

 marine animals, we find them to disappear separately 

 at several successive zones of strata, so that for each 

 class we have reason to think the origin in time is 

 reached, or at least approached. Thus Fishes are 

 not found below the Ludlow Rocks; Rhynchonella 

 and Avicula hardly pass below Caradoc beds ; Dimy- 

 aria and Cephalopoda grow very rare in the Llandeilo 

 Rocks ; and Lingula is almost the solitary occupant 

 of the lowest laminae of the fossiliferous beds of 

 Wales. 



No doubt it is open to any one to compare this 

 approach to a Hypozoic Zero, with the reductions of 

 life to a minimum above the Palaeozoic and above 

 the Mesozoic deposits; and to suppose that below 

 the Palaeozoics were other earlier strata and earlier 

 systems of life, though they are now all lost in the 

 general metamorphism which has produced the 



