ADAPTATION 353 



which is essentially fickle and inconstant. We believe that the 

 degree of adaptation is best measured by the power conferred 

 over the environment. All living organisms are, of course, 

 intimately related to their environment, but one or the other 

 partner in the relation may ' call the tune.' 



In its relation to environmental pressure the organism may 

 take one of three courses : (1) Modification, (2) Compensation, 

 and (3) Independence. 



(1) Modification implies that subservience to the environ- 

 ment which we have already considered under specialisation. 

 In a highly specialised and relatively uniform environment 

 great temporary success may result from it, but with changing 

 conditions it may mean annihilation. As will be seen later, 

 this applies more especially to animals living in habitats to 

 which only a limited number of responses are possible. 



(2) Compensation is a fundamental property of living matter. 

 An organism without the power of adjusting itself to changes 

 in the environment could not maintain itself as a living entity. 

 The essence of modifications is that, though they allow the 

 organism to survive, they mortgage its future and reduce its 

 liberty of action. Compensations allow the organism to con- 

 tinue its old types of behaviour, although the environment has 

 altered. The simplest type of compensation is perhaps seen 

 in migrations from one part of a habitat to another ; the most 

 complex in such phenomena as the control of the pH of 

 mammalian blood. 



(3) Independence is perhaps only an ideal, but it is one 

 towards which an organised system of compensations is 

 evidently leading. A completely independent organism would 

 respond to all possible changes in the environment by self- 

 regulation. In certain features and within certain limits most 

 animals exhibit independence, the development of which is 

 one of the most obvious characters of the evolutionary hierarchy. 



In the following paragraphs we shall further expand this 

 argument with a number of examples. Finally, we shall con- 

 sider the very difficult question of the relation of modification 

 to compensation in the course of evolution. 



The simplest type of compensatory response is seen in 

 many aspects of animal behaviour. The comparison between 

 structure and behaviour, as regards their power of response, 

 illustrates this point. In the case of structure, this power is 



2 A 



