GENERAL SUMMARY AND CONCLUDING REMARKS 



the more reason is there for its survival. The reactions that 

 we meet with in bird life may thus be said to have been 

 developed firstly as a means for arousing the requisite amount 

 of pairing hunger in the female, secondly as a warning to 

 intruders, and thirdly as a protection for the helpless offspring. 

 Clearly everything centres round the strength of the probability 

 in each case. 



Emotional behaviour as a whole is so intricate that it is 

 difficult to lay hold on anything very definite, but observation 

 discloses one fact at least which seems to be of some impor- 

 tance and which is referred to in the life of the Marsh Warbler, 

 namely, that the visible manifestations are uniformly strong or 

 uniformly weak in the same species at different emotional 

 periods. An example will make my meaning clearer. The 

 Blackcap exhibits many peculiar antics and assumes many 

 peculiar attitudes during the period of sexual activity and again 

 when it has young. The Willow Warbler assumes one peculiar 

 attitude during sexual activity and again, though in a minor 

 degree, when its nest or young are intruded upon. Now the 

 sexual behaviour of these two species is widely divergent ; the 

 one bird is intensely demonstrative, no limit being placed 

 apparently upon the antics it can and does perform, the other 

 has only one definite performance which really calls for remark. 

 Great however as the difference is between them, it need not 

 trouble us here, since a certain amount of latitude is permiss- 

 ible in the application of the utility hypothesis to the behaviour 

 at this special time ; let us therefore grant that in each case 

 it serves its purpose equally well in its own particular sphere- 

 arouses, that is to say, the pairing hunger of the respective 

 females. Our difficulty arises in regard to the behaviour when 

 the parental instinct is uppermost ; for it is then no longer a 

 question of calling forth a response in this female or in that, 

 but of influencing the behaviour of an intruder. A certain 

 definite standard of response is consequently necessary in the 

 case of both species, since a very definite part has to be played, 

 and this standard must have been gradually evolved by 



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