64 LIFE ON THE EARTH. 



In this diagram the Passage Beds are taken at 

 500 ft. with 13 species, Ludlow 2000 ft. with 228 spe- 

 cies, Wenlock 2500 ft. with 322 species, Llandovery 

 2500 ft. with 196 species, Caradoc 6800 ft. with 335 

 species, Llandeilo 6800 ft. with 104 species, Lingula 

 beds 4000 ft. with 14 species. The area of the spaces 

 corresponds to the number of species, thus shewing 

 at a glance the relative richness in species of the 

 several groups for equal thicknesses. This richness 

 (expressed by numbers on the right side) rises from 

 almost zero in the Lingula beds to a maximum in 

 the upper part of the Wenlock series, and dies out 

 almost to zero in the beds of Passage to the Old Red 

 Sandstone system above. 



What is here said of the Siluro-Cambrian or 

 Lower Palaeozoic Strata may be repeated with equal 

 truth in reference to each of the systems of associated 

 deposits : for in each the characteristic and prevalent 

 fauna begins at a minimum, rises to a maximum, and 

 dies away to a final minimum, to be followed by an- 

 other system having similar phases. The most remark- 

 able and prevalent of these surfaces or zones of least 

 life are those two which separate the Palaeozoic from 

 the Mesozoic, and these from the Caenozoic Series. 

 The Palaeozoic Series dies out through the Permian 

 system, and the Mesozoic rises slowly in the Trias; 

 so the Mesozoic Series dies away in the uppermost 



