EXCURSIONS OF AN EVOLUTIONIST 



epoch extends into the sixth aeon, and then 

 comes the Silurian, which takes us halfway 

 through the seventh. Mollusks and crustaceans 

 swarmed in the seas of the Cambrian epoch, 

 but there are as yet no traces of fish before the 

 upper Silurian. That is to say, three fifths of 

 the whole duration of geological time had 

 elapsed before the lowest vertebrate forms had 

 begun to leave plentiful traces of themselves in 

 the rocks. The Devonian epoch, in which we 

 find the first record of insects, carries us half- 

 way through the eighth aeon ; and we are brought 

 well on into the ninth by the Carboniferous age, 

 in which appear the earliest air-breathing ver- 

 tebrates in the shape of frog-like amphibians. 

 The vegetation of this period consisted chiefly 

 of ferns, club-mosses, and horse-tails with arau- 

 carian pines. Nearly nine tenths of the past life 

 history of our globe accomplished, and as yet 

 no birds or mammals, perhaps no true reptiles, 

 nor any tree save the araucaria or the arbores- 

 cent fern ! With the Permian epoch we reach 

 the end of the Primary period, and nearly com- 

 plete our ninth aeon, leaving for the whole of 

 the Secondary and Tertiary periods only a little 

 more than one aeon to be divided between them. 

 The oldest mammals and reptiles yet found 

 belong to the Trias, or earliest Secondary epoch ; 

 yet so many small mammalia, inferior in type 

 to the marsupials, have been found by Professor 

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