302 PALEOZOIC SYSTEM OF ROCKS. 



and highly-organized types originated together at once. "We must re- 

 member that between the Archaean and Palaeozoic there is a lost inter- 

 val of enormous duration. Evidently, therefore, the Primordial fauna 

 is not the actual first fauna. Evidently we have not yet recovered the 

 leaves in which is recorded the gradual differentiation of these widely- 

 distinct types. All this must have taken place during the lost in- 

 terval. 



But if, on the other hand, we suppose, as many do, that evolution 

 proceeds always " with equal steps," then we are forced to the very im- 

 probable conclusion that the lost interval is equal to all geological times 

 which followed to the present ; for the differentiation of types which 

 occurred during that interval is equal in value to all that has taken 

 place since. 



Therefore, we are compelled to admit that there have been in the 

 history of the earth periods of rapid change in physical geography, and 

 periods of comparative quiet in this respect ; that, corresponding with 

 these, there have been also periods of rapid evolution of the organic 

 kingdom, developing new forms, and periods in which forms are more 

 stationary. The periods of rapid change are marked by unconformity, 

 and are therefore unfortunately often lost. 



As we proceed, we will probably find many examples of rapid change 

 which must be accounted for in a similar manner. 



General Life-System of the Silurian Age. 



After this rapid sketch of the first fauna, we now take up the gen- 

 eral life-system of the whole age. 



There were evidently extraordinary abundance and variety of life 

 in the Silurian. These early seas literally swarmed with living beings. 

 The quantity and variety of life — the number of individuals and of 

 species — were probably not less than at the present time ; though or- 

 ders, classes, and departments, were less diversified. Over 10,000 spe- 

 cies have been described from the Silurian alone (Barrande) ; and 

 these must be regarded as only a small fragment of the actual fauna 

 of the age. In certain favored localities, the number of species found 

 in a given area of a single stratum will compare favorably with the 

 number now existing in an equal area of our present sea-bottoms. Yet, 

 in all this teeming life there is not a single species similar to any found 

 in any other geological time. And not only are the species peculiar, 

 but even the genera, the families, and the orders, are different from 

 those now existing. 



We can give only a very brief sketch of this early life, touching only 

 the most salient points, especially such as throw light on the great 

 question of evolution. 



