THE APPEARANCE OF LIFE ON THE GLOBE 327 



Nebalias, and especially by the then flourishing group 

 of Trilobites. Of about 150 known genera of this 

 group, one third or 50 genera with 250 species, have 

 been discovered in the Cambrian system. The 

 Trilobite fauna of the Cambrian is entirely distinct 

 from that of the Silurian, but a very small number 

 of genera, such as Agnostus and Conocoryphus, pass 

 into the Silurian fauna. Contrary to what we 

 noticed in the Silurian epoch, no trace of Verte : 

 brates has yet been seen in Cambrian strata ; and 

 it seems logical to consider the absence of the 

 Fishes in the seas of that epoch as simply a tem- 

 porary gap in our discoveries. 



The Cambrian world, therefore, shows itself, the 

 Vertebrates apart, to be largely constituted of the 

 same general elements as the Silurian world. All 

 the great groups of Invertebrates are there repre- 

 sented, and by the most highly organized types, 

 such as the Cephalopods in the branch of Molluscs, 

 and the Crustacea in the branch of Articulates. 

 May we not suppose without any great improb- 

 ability that these animals, so complex and so 

 advanced in evolution to use the transformist 

 language, have been preceded by more simple 

 ancestors, belonging to the lower groups of the same 

 branch ? The material answer to this question has 

 only been supplied within a very few years. 



We have not, in fact, yet terminated our journey 

 into the depths, in search of the first traces of 

 life in the thickness of the sedimentary crust of the 

 Earth. Beneath the Cambrian soil there exists in 

 different countries, in France, in England, in Fin- 

 land, and in North America, enormous masses of 



