34 BRITISH BIRDS. [vol. viii. 



of fixed and fundamental instincts, and the other of 

 indeterminate behaviour-tendencies readily modifiable in 

 the individual, stand certain hereditary dispositions which, 

 Avhile not, or scarcely, accommodative in the individual, 

 are yet racially plastic as compared with the more rigid 

 forms of behaviour. If this be so, then adaptation in 

 the one class may precede it in the other. There will 

 be a sequence of adaptation. In manner as the varied 

 " requirements " of a species, which are the objectives 

 of its mental constitution, govern distribution in pro- 

 portion to their specialization, so fimctional adjustment 

 will tend to be limited initially by the most specialized 

 or conservative character. 



It is a characteristic common to all life -forms that the 

 activities related to reproduction display a greater stability 

 and conservatism than the remaining life -f mictions — a 

 conservatism extending to the physiological and mental 

 connexions in direct ratio to the intimacy of their 

 relation. The general tendency appears to be clearly 

 exemplified in birds — in life-history, psychology, and in 

 distribution. Birds are most local, ^^-hether in space, or 

 in relation to habitat-tj^e, during the breeding-season. 

 It is no explanation — it is a re-statement of the facts, 

 to say that the tendenc}^ is due to the fact that Avhile 

 the extra-breeding range may be co-extensive with the 

 food-supply, the breeding-season necessitates the co- 

 existence of food-supply and nesting-ground ; or that the 

 food -requirements of the young involve localization of the 

 breeding-area. If the environmental range in the nesting- 

 season is restricted, and the food and other requirements 

 of the young specialized, does not this merely illustrate 

 a relative functional specialization in relation to the 

 central reproductive activities ? The frequency A\'ith which 

 the nesting -ground requirements act as checks to exten- 

 sion or increase, is essentially bound up with this 

 tendency. Within the species, that group of activities with 

 narrowest environmental adjustments necessarily limits spread 

 or increase. If the environmental limits within A\'hich 

 reproduction in birds is confined are narrower than those 

 of other functional adjustments, then the former will be 

 in proportionate degree the limiting factor in dispersal. 

 If this suggestion is well founded, then in species with 

 differentiated environmental range {i.e., species part of 

 whose extra breeding habitat differs in character from 

 that of the nesting-area), the nesting -groimd should on 

 the average represent the original centre of distribution, 



