90 THE FAR PAST. 



which marks the geological periods. In this opinion we are 

 further fortified by the decidedly expressed conviction of one 

 of the ablest investigators of the present age. " However 

 much naturalists may still differ in their views regarding the 

 origin, the gradation, and the affinities of animals," says Pro- 

 fessor Agassiz in his Essay on Classification, " they now all 

 know that neither radiata, nor molluscs, nor articulata have 

 any priority one over the other as to the time of their first 

 appearance upon earth j and that, though some still main- 

 tain that vertebrata originated somewhat later, it is univer- 

 sally conceded that they were already in existence towards 

 the end of the first great epoch in the history of our globe. 

 I think it would not be difficult to show, upon physiologi- 

 cal grounds, that their presence upon earth dates from as 

 early a period as any of the three other great types of the 

 animal kingdom, since fishes exist wherever radiata, mol- 

 luscs, and articulata are found together, and the plan of 

 structure of these four great types constitutes a system 

 intimately connected in its very essence. Moreover, for the 

 last twenty years every extensive investigation among the 

 oldest fossiliferous rocks has carried the origin of vertebrata 

 step by step farther back j so that, whatever may be the 

 final solution of this vexed question, so much is already 

 established by innumerable facts, that the idea of a gradual 

 succession of radiata, molluscs, articulata, and vertebrata, is 

 for ever out of the question." 



Here, then, in the silurian system we find nothing ab- 

 normal or marvellous ! Its sediments tell of seas whose 

 shores, in favourable localities, were clad with weeds, and 

 whose waters were thronged with zoophytes, star-fishes, 

 sea-urchins, shell-fish, and Crustacea. Plant-feeder and 

 animal-feeder start simultaneously in the race of life ; and 

 it requires no great stretch of fancy to repeople silurian 

 waters, busy and joyous on a summer's eve as the tribes 



