46 THE ECOLOGY OF ANIMALS 



variable ? Again, there is a growing body of evi- 

 dence that wild animals do not consistently frequent 

 one habitat. Changes in habit are frequent, and we 

 do not as yet know precisely what relative impor- 

 tance to attach to psychological factors (new ideas, 

 or broken traditions or cumulative fatigue with old 

 habits) and how much to organic changes in the 

 form of mutations affecting behaviour. Finally, it 

 is of great interest to inquire whether animals are 

 actually conscious of their actions, and whether in 

 this consciousness there is any element which is at 

 variance with the usual concepts of animal behaviour 

 current among physiologists and also many ecolo- 

 gists ? There is definite evidence that animals often 

 migrate in response to stimuli which cannot be 

 called danger signals but which appear to be simply 

 unpleasant to them (Elton, 1930). Whether in this 

 behaviour we can discern feelings akin to aesthetic 

 feehngs, or whether they are to be looked upon as 

 mechanical upsets of mental balance, cannot be 

 decided. The whole question of animal behaviour 

 in relation to the choice of habitats and habits in 

 general is of profound importance both in theoretical 

 science and in practical economic biology. For in- 

 stance, the herring shoals during their migration in 

 the North Sea avoid certain patches of sea polluted 

 by the colonial plankton flagellate {Phaeocystis 

 poucheti), and the diatom {Rhizosolenia sinensis) 

 (Savage, 1932). The variable position of these 

 patches is an important factor in determining the 

 success of East Anglian fisheries at certain times of 

 year. The changing of its food plant by an insect 

 may produce a new pest with far-reaching results. 

 The kea of New Zealand changed from catching 

 insects to scratching out the kidneys of sheep, and 

 created an economic problem. A bird may change 

 its reactions to habitat and by living in a new 

 habitat alter the whole trend of evolution in its 

 structure. 



