ENVIRONMENT, PAST AND PRESENT 87 



nebulous hypothesis but even if unsatisfactory it 

 might yet serve a useful purpose. Migrating birds 

 and a suitably controlled magnetic field brought 

 together would provide a combination with rather 

 interesting experimental possibilities. 



If the fully established migrations with which we 

 are so familiar today in the northern hemisphere 

 depend neither on a bird's personal experience nor 

 on a conscious knowledge of the experience of its 

 ancestors or the factors of its environment, their 

 seasonableness, precision and accuracy, on the other 

 hand, can leave little doubt that they hinge in some 

 other way on the experience of past generations. 

 Such experience has not been perpetuated by word 

 of mouth or in writing and we must therefore as- 

 sume that it has been handed on by another method, 

 — genetically, by inheritance. We must assume 

 that the habit has been acquired by individuals of 

 the past, that it has somehow become inherent and 

 that it has survived because it remains of value or 

 is even essential, today. In short, it is now instinc- 

 tive. 



Instinctive behaviour may be defined thus in the 

 words of Lloyd Morgan^ — "those complex groups of 

 coordinated acts which are, on their first occurrence, 



^ Animal Behaviour, London. 



