600 HISTORICAL GEOLOGY. 



complex structure. The latter is simply an exhibition of the gene- 

 ral law of succession in the creations by which the system of life 

 reached its completion. 



2. Eelation of the History of Life to the Physical History of the 



Globe. 



1. The plan of progress was determined with reference to the last age, with 

 all its diversities of climate, continental surfaces and oceans, as its era of 

 fullest exhibition. 



2. The progress in climate and other conditions involved a concurrent pro- 

 gress from the inferior living species to the superior. — The existence of a 

 long marine era, through the Silurian and part of the Devonian 

 ages, admitted only of the existence of marine life. Hence the 

 dominant type of the Silurian was the Molluscan, which, with the 

 Eadiate, is eminently marine. In addition, there were marine Arti- 

 culates and marine plants ; and when the Vertebrates began it was 

 with marine species, the Fishes. Thus the prevalence of waters 

 involved inferiority of species. The increase of land, gradual purifi- 

 cation of the atmosphere, and cooling of the globe, prepared the way 

 for the higher species. 



It is probable that the oceanic waters were also in an impure state 

 compared with the present, from containing an excess of salts of 

 lime; and this also involved the existing of inferior species, — such 

 as Crinoids, Corals, and Mollusks, a very large proportion of whose 

 weight is in calcareous material. The removal of this excess of 

 lime from the waters produced limestone strata, purified the 

 waters, and fitted the oceans for other species. 



The great prevalence, in the Primordial, of Lingulae (whose shells contain a 

 large amount of phosphate of lime) is further evidence of the greater density 

 of the waters, and seems to indicate the presence of an excess of phosphates. 



3. The progress in climate and in the condition of the atmosphere and waters 

 involved a localization of tribes in time, or chronographically, just as they are 

 now localized by climate over the earth's surface, or geographically. — Tribes 

 were made for a special climate or condition of the globe ; and when 

 this climate or condition had been passed in the earth's progress, 

 the tribes no longer existed. The culmination of the Eeptilian 

 and Molluscan types in the Eeptilian age, or of Trilobites and 

 Brachiopods in Palaeozoic time, are examples. The former when in- 

 stituted had those special relations to climate that made the Eepti- 

 lian age the era of their culmination ; just as now palms and bananas 

 reach their perfection only in the equatorial zone ; figs in the 



