THE PROGRESS OF LIFE 69 



Radiolarians, 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 necessarily 

 follows that there is no inherent necessity for organisms 

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

 they must subsequently decay, is opposed to the 

 whole teaching of organic 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 the latter half of the 

 earth's history, the struggle for existence has been 

 so complicated that it is hardly possible to trace out 



