THE MEANING OF EVOLUTION 261 



throughout all depths, and in almost all physical con- 

 ditions in the sea. Some species of marine Mammalia 

 and Birds are quite cosmopolitan except that they are 

 restricted to the upper layers of the ocean. Land 

 Mammals are subject to the same restrictions as are 

 the green plants, being unable to survive in desert and 

 polar areas. The only parts of the sea which are not 

 inhabited by Arthropods and Vertebrates are those 

 limited deep strata of water (as in the case of the deeper 

 layers of the Black Sea) where there are accumulations 

 of poisonous chemical substances in solution. But the 

 Bacteria inhabit even these regions. 



Green plants, Arthropods, and Vertebrates appear 

 as fossils in almost every part of the stratified rocks. 

 The Trilobites represent the end of a long evolutionary 

 process, and the same is to be said of the first fishes 

 found in Silurian rocks, so that these groups of animals 

 must have existed in the geological periods represented 

 by those remains of rocks which are older than the 

 earliest fossiliferous ones. Plant remains are present 

 in Silurian rocks, but there can be no doubt that Ferns 

 and other chlorophyllian organisms must have been in 

 existence long before this time. We can hardly suppose 

 that the Bacteria found in the Carboniferous rocks 

 first appeared at this time in the earth's history : like 

 the other great groups of life they probably had a pro- 

 longed history prior to that date of the geological 

 formations in which they are first to be recognised. 

 Our dominant groups of organisms may therefore be 

 traced back almost to the very beginnings of life on 

 the earth. 



Dominance, such as we have defined it, cannot be 

 said to have been attained by any other of the sub- 

 kingdoms of life. Ccelenterates and sponges appear to 

 have existed throughout the whole period during which 



