VOL. VIII.] RELATIONS OF BIRD-DISTRIBUTION. 39 



supply of nesting-sites where through relative insufficiency 

 the latter is the limiting factor in distribution. But it is 

 probably also in some instances modified or checked through 

 constitutional disposition as expressed in intolerance or 

 unsociability, which mental attributes are not necessarily 

 perfectly attuned to present conditions of existence ; that is 

 to say, an intolerant disposition, \\hich is the reflection of 

 keen ancestral competition, may demand present territory 

 the dimensions of which exceed present needs — and so far it 

 is a survival rather than a present adjustment. 



It is obvious that the quantitative relation between food- 

 supply and that of nest-sites is important in determining the 

 form of dispersal of the association-units ; and this relation 

 is probably a leading factor in the relative sociability of 

 breeding birds. It is open to observation that birds as a 

 whole show most sociability outside the breeding-season. 

 The rivalry and jealousy coincident with the seasonal effer- 

 vescence of the sexual emotions are reflected in a wide -spread 

 tendency to exclusiveness during the nesting-period. Other 

 things equal, birds then tend to disperse themselves separ- 

 ately and evenly. The inclination towards sociability is in 

 proportion to the superabimdance or wide dispersal of the 

 food -supply as contrasted with a comparative localization of 

 nest-sites. That the genesis of the colonial liabit is at least 

 in part related to this factor is rendered the more probable 

 by the negative evidence as to the existence of true social 

 links and sub -division of labour within the colony. The 

 colony is in origin probably in large part rather a forced aggre- 

 gation than a true society : it is the result of compression from 

 A^ithout rather than of attraction from within. But it is 

 credible that a sociability so acquired may become in time 

 independent of the environmental peculiarities which origin- 

 ated it. 



The alteration and impoverishment of the British avifaima 

 within historic times is no doubt essentially attributable to 

 human interference ; but great as has been the effect of 

 direct persecution and extermination, the indirect result of 

 man's destruction and modification of primitive ground has 

 probably been infinitely greater. Persecution has locally 

 exterminated individual species ; environmental alteration 

 has locally caused the disappearance of whole groups of 

 species. The felling of a forest or the draining of a marsh 

 is more effective than much slaughter. Oiar avifaima has 

 been much modified by man — in considerable degree in its 

 faimistic composition ; profoundly in its dispersal. A 

 reconstructed picture of its primitive aspect must he based 



