xii. 28 BEHAVIOUR 355 



The search for food and the avoidance of enemies are not in prin- 

 ciple more difficult on land than in the water, but they probably 

 demand new mechanisms. For example, the greater range of visibility 

 can be a disadvantage, especially when it is exploited by one's success- 

 ful and predatory descendants. A hawk or owl makes fuller use of its 

 opportunities in this respect than does the frog, who can only remain 

 safe from them by behaviour that keeps it concealed. Similarly there 

 are dangers in certain situations, for instance of desiccation, which 

 are additional to those that are met by an animal in the water. 



In the emergence of the first land vertebrates we thus see a con- 

 spicuous example of the invasion by living things of a medium far 

 different from themselves. This produces a situation that calls forth 

 all the powers of the race to produce new types of individual, and 

 necessitates that the individuals make full use of their capacities. 

 New patterns of structure and behaviour are developed as the various 

 possible situations emerge. The types of organization that at first 

 manage to survive gradually give place to others, still more complex 

 or 'higher'. Some traces of the organization of the early venturers can 

 still be seen in the amphibia, which today exploit the damper situa- 

 tions on the earth. 



