ENVIRONMENT, PAST AND PRESENT 83 



precise return to their accustomed stations. In 

 northern waters birds have frequently been noted 

 flying back to their breeding cliffs in thick fog, ap- 

 parently with as much certainty as though visibility 

 had been perfect. Such birds are homing; they are 

 returning to a known site. 



In the juvenile migrations of many species we en- 

 counter something fundamentally different. The 

 individual traverses enormous tracts of country 

 that it has never before seen. Sun, stars and land- 

 marks can mean nothing to it. Memory, based on 

 personal experience, can be no part of the achieve- 

 ment. This is migration in its most striking form. 

 If we attempt to analyse it we run into one obstacle 

 after another. 



If we term it instinctive behaviour, as we must, 

 we are assuming that the bird is profiting from the 

 experience of its ancestors, that it has what might 

 be termed ''inherited memory." This difficulty 

 applies to all examples of instinctive behaviour, not 

 solely migration. But of inherited memory, scienti- 

 fically speaking, we know nothing. We can define 

 instincts, (p. 87) but of the essential laws under- 

 lying them we are ignorant. If w^e adopt the 

 Lamarckian viewpoint and assume that by con- 

 stant repetition, generation after generation, a par- 

 ticular habit can ultimately become hereditary, 



