92 THE THEORY OF EVOLUTION 



than any living representatives of those groups, 

 and groups which now are very rare were then ex- 

 tremely abundant, and many were dominant in the 

 Palaeozoic. For example, molluscs were less com- 

 mon and brachiopods and sea-lilies vastly more so 

 then than now. Confined to the Palaeozoic were 

 the remarkable Trilobites, an extinct sub-class of 

 the Crustacea, while the higher groups of the 

 latter were wanting. Spiders and scorpions were 

 followed by insects belonging to the lower and more 

 primitive divisions, such as cockroaches and dragon- 

 flies. 



The species of Palaeozoic fossils already described 

 are numbered by thousands and the number is 

 constantly growing, as new discoveries are made. 

 Any attempt to survey this vast assemblage is a 

 highly technical problem and cannot be made here. 

 The few salient features which have been selected 

 are enough to show that the farther back in time 

 we explore the history of life, the greater do the 

 differences from the present order of things become 

 and that not in any haphazard way, but by changes 

 in a well-defined serial order. The history of life, 

 both animal and vegetable, is a story of progress and 

 differentiation, of advance continued through mil- 

 lions of years to modern conditions from far-off 

 beginnings, which were of radically different charac- 

 ter. In itself, this story is a striking testimony to 

 the theory of evolution and the testimony is much 

 strengthened, when the material at hand permits a 



