HABITATS 45 



ever, an enormous class of animals which have 

 special reactions for finding their normal habitat. 

 This finding may take the form of not going outside 

 the habitat they are in (avoiding the wi'ong one), or 

 of seeking new areas of the normal habitat by local 

 movements (e.g. butterflies laying eggs on certain 

 plants) or by long-distance migrations (e.g. woodcock 

 choosing certain woods for breeding in summer). 

 The second question is whether such actions are 

 adaptive ? By this is meant whether animals always 

 choose to live in the habitat to which they are best 

 suited, or whether they choose their habitat for 

 other reasons. This question is difficult to answer, 

 and twenty years ago, in the light of the evolution 

 theories of the day, would hardly have required an 

 answer. There is, however, a small but growing 

 body of evidence pointing to the existence of impor- 

 tant plprriP^ts of oV\ oice which dep end as much upon 

 purely psycholo gical factors as upon ecologica llv 

 adaptive reasons ! Mention may be made of the 

 experiments of Thompson and Parker (1927) who 

 showed that certain polyphagous insect parasites do 

 not choose hosts according to any simple ecological 

 reasons that could be detected by the experiments, 

 which were very comprehensive. Lack (1933) has 

 brought forward important evidence that many birds 

 choose their habitats for reasons entirely uncon- 

 nected with survival or breeding limits. He shows 

 quite convincingly that some Passerine birds adopt 

 ccj tain habitats because they prefer them and for 

 no other reason. The gist of this work is that 

 animals probably do not usually inhabit ail the 

 places which they could inhabit if they had different 

 ecological reactions. Against this it may be said 

 that we do not yet know enough about the limiting 

 factors of the lives of animals to make a negative 

 statement of this kind. The third question, one of 

 the greatest importance in practice, is whether such 

 reactions remain constant or whether they are 



