RELATIONSHIPS IN THE ENVIRONMENT 123 



effective pairing situation, it is serviceable in 

 furthering the life of the individual. I do not 

 propose at the moment to inquire whether this 

 doctrine be true, but rather to direct atten- 

 tion to other ways in which the song may be 

 useful. 



Is the instinct susceptible to stimulation 

 under all conditions during the season of repro- 

 duction, or only under some well defined 

 condition ? This is the question to which we 

 will first direct inquiry. 



Song in its full development belongs, as we 

 have seen, to the season of reproduction ; it is 

 heard at the dawn of the seasonal sexual process, 

 and is the most conspicuous outward manifesta- 

 tion of the internal organic changes which 

 ultimately lead to reproduction. These changes 

 would appear, at first sight, to be the primary 

 condition which renders the instinct, susceptible 

 to appropriate stimulation. But while this is 

 true up to a point, in so far, that is to say, as 

 organic changes are a necessary antecedent of 

 all behaviour connected with the attainment 

 of reproduction, closer acquaintance with the 

 circumstances under which the instinct is allowed 

 full play leads to the belief that they are not 

 alone sufficient to account for the facts as 

 observed. In order to arrive at a decision we 

 must seek out the specific factors in the external 

 environment with which "song" is definitely 

 related. 



Some birds cross whole continents on their 

 way to the breeding grounds, others travel many 



