THE LOWER AND UPPER SILURIAN AGES. 77 



was no animal life on the land ; but here our knowledge 

 may be at fault. It is, however, interesting to observe 

 the continued operation of the creative fiat, "Let 

 the waters swarm with swarmers," which, beginning 

 to be obeyed in the Eozoic age, passes down through 

 all the periods of geological time to the " moving 

 things innumerable " of the modern ocean. Can we 

 infer anything further as to the laws of creation from 

 these Silurian multitudes of living things ? One thing 

 we can see plainly, that the life of the Silurian is 

 closely related to that of the Cambrian. The same 

 generic and ordinal forms are continued. Even some 

 species may be identical. Does this indicate direct 

 genetic connection, or only like conditions in the 

 external world correlated with likeness in the organic 

 world ? It indicates both. First, it is in the highest 

 degree probable that many of the animals of the Lower 

 Silurian are descendants of those of the Cambrian. 

 Sometimes these descendants may be absolutely 

 unchanged. Sometimes they may appear as distinct 

 varieties. Sometimes they may have been regarded 

 as distinct though allied species. The continuance in 

 this manner of allied forms of life is necessarily related 

 to the continuance of somewhat similar conditions 

 of existence, while chang-es in type imply changed 

 external conditions. But is this all ? I think not ; 

 for there are forms of life in the Silurian which cannot 

 be traced to the Cambrian, and which relate to new 

 and even prospective conditions, which the unaided 

 powers of the animals of the earlier period could 



