LIFE ON THE EARTH. 



57 



lower parts of the Palaeozoic Strata, Zoophytes are 

 introduced (Z) instead. We have added also a column 

 for the Cephalopoda (Cc). 



In the same manner we may select genera which 

 occur in the older strata, and trace upward those 

 which have the greatest range in the strata, in other 

 words, the longest duration in time. 



Again, if we consider not genera but greater 

 groups as families, their extension is greater, their 

 antiquity higher ; while some classes of the radiated 

 and molluscous and articulated divisions occur in 

 all the strata. Thus the general result is to shew 

 that only one general plan of creation is to be traced 

 in all the range of geological time, though the terms 

 in which this plan is expressed vary from period to 

 period. There is a succession of the forms of life to 

 be traced in the strata ; we shall shew hereafter that 

 the relative proportions of the families, orders, and 



