256 WARFARE BETWEEN DIFFERENT SPECIES 



highway is formed by the bird passing constantly 

 to and fro. But the attention which it pays to 

 other species is very different on these two 

 occasions — when in the territory it is intolerant 

 of strangers, but when it accompanies the flock 

 it displays no interest in their movements. 

 From hour to hour its nature seems to change. 

 But, as we saw, the inborn constitution of the 

 bird cannot change, and therefore we came to 

 the conclusion that an explanation of the altered 

 behaviour was to be found in the fact that the 

 pugnacious instinct is only rendered susceptible 

 under a certain condition. So that all the 

 evidence tended to confirm the impression which 

 we had gained from the course of events in the 

 life of the migrant, namely, that the hostility 

 bears a direct relation to the occupation of a 

 territory. 



Finally we were led to inquire whether the 

 hostility was serviceable in promoting the 

 welfare of the individuals. We saw that many 

 different species require similar conditions of 

 existence, that ancient breeding haunts dis- 

 appear and that new ones come into being, and 

 that in the ordinary course of events such 

 species must often assemble in the same area for 

 the purpose of reproduction. So that even 

 though a male might be successful in protecting 

 its ground from intruders of its own kind, yet it 

 might still fail to rear offspring, just because it 

 happened to choose a position in which other 

 kindred forms had gathered. Hence if the 

 territory is adequately to serve its purpose. 



