^L. VIII.] RELATIONS OF BIRD-DISTRIBUTION. 43 



;he racial environment— which latter the bird may be said 

 :o inherit. 



The individual species depends for its existence upon the 

 [ocal supply of certain environmental imits ; and since the 

 conjunction of these is indispensable, dispersal and population 

 ttill be directly controlled by that essential which is locally 

 least efficient in supply ; that is to say, such envhonmental 

 onit will assume the position of limiting factor in distribu- 

 tion. But the relative limitmg influence of external factors 

 in distribution is ultimately in considerable degree referable 

 to a corresponding functional power of adjustment in the 

 species. There is reason to believe that the non-plasticity 

 and conservatism of the group of activities surrounding 

 reproduction, extend to the environmental adjustments 

 linked with the reproductive period. But whatever the 

 origm of the tendency, if it be m fact true that the breeding- 

 environment is relatively specialized ; that is to say, if the 

 environmental adjustments of the reproductive group of 

 activities are confmed within relatively narrow limits— then 

 the breeding -ground requirements will limit distribution 

 proportionately. In cases where the habitat to which 

 breeding is confmed is m type more uniform than is the 

 total range of the species, the former may be regarded as 

 the more primitive envnonment — a conclusion also suggested 

 by the probability that the recapitulation of racial structural 

 history appearing in the development of the mdividual will 

 be accompanied by a correlated racial envhonmental bistors' 

 reaching functional expression in the individual life-history. 

 The sequence of environmental adjustment in the individual 

 should illustrate approximately the sequence of adjustment 

 in the race. . 



The strength of the correlation between bird-distnbution 

 and that of vegetation-type is shown by the fact that the 

 breeding-avifaunas supported by different plant-associations 

 are related to one another in parallel degree as are the latter. 

 Such correspondence in distribution between bhds (in the 

 nestmg-season) and vegetation is probably more complete 

 than is that between bird-distribution and food-supply — 

 that is to say, to take an illustration : the distribution of 

 moorland bhds at that season is not co -extensive with the 

 food-supply beyond moorland lunits, although an available 

 food-supply (presumably) exists beyond such lunits. Such 

 environmental restriction during the nestmg-season is 

 apparently attributable to the environmental conservatism 

 of the reproductive and related activities. 



