106 EVOLUTION OF LIVING ORGANISMS. 



may be useless or harmful, and doubtlrss these unsuc- 

 cessful forms may have got into grooves of variation ; 

 but variation is not evolution. And seeing that natural 

 selection looks not to a distant future but to the immediate 

 advantage, there is nothing in the history of these animals 

 which cannot be explained as due to the ordinary action 

 of selective elimination. We have every reason to sup- 

 pose that every step in increase of size gave some advan- 

 tage to the giant forms over their competitors. To assume 

 the contrary would be as wise as to argue that the huge 

 modern Dreadnoughts are useless ships of war because 

 they may possibly be driven off the seas by the relatively 

 small submarines and flying machines, or to deny that 

 the progressive stages in the development of these men- 

 of-war must each have surpassed in usefulness those 

 which went before. 



This brings us to another interesting subject on which 

 palaeontology can throw some light namely, the rate of 

 evolutionary change. Some organisms have changed 

 very little through long geological periods. The mol- 

 luscan genus Nucula, and the genus Patella, which in- 

 cludes our common limpet, date back to the Silurian 

 epoch ; the Silurian scorpion (Palseophonas) differs little 

 from modern forms ; the Brachiopod Lingula has 

 changed very little since the Ordovician ; and many 

 Protozoan skeletons are found in Cambrian rocks which 

 differ but little from those of the present day. There 

 can be no doubt, then, that the rate at which the various 

 branches of the phylogenetic tree have grown varies 

 very much. Presumably the persistent types have been 

 sufficiently well adapted to keep their own place against 

 competitors, though perhaps only where the struggle 

 has not been very severe. The fact that they have 

 changed little does not in the least prove that they have 

 not varied, but merely that the divergent variations have 

 not been selected. If variations are eliminated as fast 

 us they occur, an organism may continue unchanged for 



