282 RELATION OF TERRITORY TO MIGRATION 



enjoyment, but all that can be definitely asserted 

 is — that past experience somehow becomes 

 ingrained in the life of the individual and 

 determines present behaviour. What, however, 

 is of importance to us at the moment is not the 

 ad hoc nature of the bird, but the biological 

 consequences to which the behaviour leads. 

 For if, on the average, individuals return to 

 their former haunts, it follows that the annual 

 dispersion will not be merely a repetition in this 

 season of that which had occvu-red in a previous 

 one, but that the little added this year will 

 become the basis for further additions in the 

 next. The innate ability is handed down from 

 generation to generation, and, in so far as it 

 contributes to success, is fostered and developed 

 by selection ; and the modifications of behaviour 

 to which it leads, since the results of prior 

 process in the parent persist as the basis and 

 starting-point of subsequent process in the 

 offspring may in a sense also be said to be 

 handed down. 



(3) The conditions in the external world may 

 be organic or inorganic. By organic I mean 

 the conditions which depend upon the number 

 of competitors or enemies by which a bird is 

 surrounded. The competitors may include other 

 species which require a similar environment; and 

 the enemies, species which prey upon it, or 

 animals which take its young or its eggs. They 

 vary in different seasons, in different districts, 

 and in nature and extent — the success of one 

 species leads to the failure of another, and the 



