776 CONCLUSION xxxn. 10- 



10. Successive replacement among land vertebrates 



It is equally easy to trace out successions of types occupying habitats 

 on land, though here it is even more difficult to be sure that the succes- 

 sive animals are occupying identical niches. There has been a long 

 series of large land herbivores, including the labyrinthodonts, pareia- 

 saurs, herbivorous synapsids, various dinosaurs, multituberculates, 

 condylarths, dinocerates, pantodonts, brontotheres, horses, pigs, 

 rhinoceroses, elephants, and artiodactyls. Clearly not all of these lived 

 in similar surroundings (and there were, of course, other herbivores), 

 but the succession is impressive. As with the aquatic animals we have 

 the curious phenomenon that the earlier members of each group seem 

 to be clumsy creatures, no better fitted for their life than those they 

 are replacing. The early mammalian herbivores, with their large limbs 

 and small brains, do not seem greatly superior to the stegosaurs and 

 ceratopsians of the Cretaceous. It is, of course, exceedingly difficult to 

 know enough to settle such questions, for instance to assess the value 

 of warm-bloodedness. 



If we look at other ecological niches we see the same picture of con- 

 tinued replacement. Thus there has been a succession of land carni- 

 vores, first synapsid reptiles, then archosaurian reptiles, followed in 

 the Tertiary period by the creodonts, which were replaced by modern 

 carnivores and carnivorous birds. 



1 1 . Is successive replacement due to climatic change ? 



A very careful analysis is needed before we can venture to say much 

 about the nature of this successive replacement of types. We have 

 several times noticed how easy and dangerous it is to find superficial 

 'causes' for evolutionary change. The periods of time involved are so 

 long that a stern discipline is needed to prevent oneself from using 

 analogies that are really only applicable to much shorter periods 

 (p. 572). We have to try to imagine vast communities of animals of 

 various sorts, interacting with each other to produce fluctuating popu- 

 lations, all living in climatic conditions that vary from year to year and 

 also, very slowly, through the centuries. Only if we hold such a picture 

 in mind can we begin to answer questions about whether the stimulus 

 to evolutionary change comes from the changing environment. 



It is now very doubtful whether there have been periods of 'revolu- 

 tionary' geological change (p. 16). Local rises and falls and foldings 

 of the crust have certainly occurred and must have influenced the 

 fauna. Even so it is not certain that the new types appearing after such 

 events, originated during them. They may well have evolved elsewhere 





