34 THE LESSON OF EVOLUTION 



the earliest times, of Eadiolarians, almost identical 

 with their descendants of the present day, is but 

 another example of the persistence of types with 

 which palaeontologists have been familiar for a long 

 time. It is true that we only know the hard parts 

 of the ancient forms ; but we have reason for 

 thinking that if the soft parts had varied much , the 

 hard parts would have changed also. From the fact 

 of the persistence of certain types it necestearily 

 follows that there is no inherent necessity for or- 

 ganisms to vary or to decay ; while the idea that, if 

 they vary they must subsequently decay, is opposed 

 to the whole teaching of biological evolution ; for it 

 is the variable groups which have progressed. But 

 if there is no internal necessity for decay, then the 

 extinction of a whole group must be due to external 

 agencies; and, if the group is widely spread, these 

 agencies cannot have been local in their operation. 



These external agencies may be changes in 

 climate, or changes in the biological environment, 

 due to the introduction of new forms of animals, 

 which may either prey on the older inhabitants or 

 be successful competitors for their food supply. 

 Change in climate may, perhaps, sometimes account 

 for the extermination of a group of terrestrial animals 

 or plants, but it cannot have a wide influence on 

 those groups which lived in the sea. These must 

 have perished either from violence or from famine. 

 The struggle for existence with other animals has, 

 no doubt, generally been the most efficient cause of 

 extinction, and with Pelagic animals it is probably 

 the only cause. At the present day, and during all 



