576 THE UNIVERSE. 



the burning surface of the globe during the plutonic period. 

 But so soon as it was sufficiently cooled down to admit 

 of living creatures appearing on it^ we see them at once 

 enter upon the scene. This is characteristic of this epoch. 



The earth, imperfectly cooled down, still maintained a 

 very high temperature, and this temperature Avas the same 

 from one pole to another; the sun only brought with it 

 useless supplementary heat. There were neither seasons 

 nor climates; the torrid zone and the polar regions were 

 peopled with the same plants and animals; their fossilized 

 remains are identical whether found beneath the ice of 

 Spitzbergen or in the rocks of burning tropical countries. 



SiLUKiAX Period. — This name is derived from that of a 

 part of England inhabited by the ancient S'dares, and is 

 given to the strata of this epoch because they have been 

 chiefly studied there. 



The globe at that time supported nothing but a very 

 small number of sea-animals, belonging to classes of the 

 lowest order of organization, as if nature, still feeble and 

 undecided, were in their production making the first trial 

 of her strength. 



The seas, still Avarm, occupied at this time nearly all 

 the surface of the globe, and only very small portions of 

 land had emerged from the waters — islets lost in the midst 

 of a boundless ocean. Crustaceans, a few scattered mol- 

 luscs, polj'poids, and a small number of fish, were the sole 

 tenants of the deep. 



But among the silurian animals, those which especially 

 predominated Avere the triloljites, the name of Avhich is 

 derived from the arrangement of their articulated bodies, 

 formed to a certain extent by three long lobes ranged side 

 by side to each other. No liA^ng representative of these 

 crustaceans, Avhich Avere the most ancient inhabitants of 



