VOL. viii.l RELATIONS OF BIRD-DISTRIBUTION. 38 



House-Martin, in using man's buildings for its nest, has 

 displayed greater initiative than the Sand-Martin ; and 

 despite the frequency with which the case is quoted as 

 an instance of modification of habit, it seems in point 

 of fact questionable wherein the suggested adaptation 

 lies. The organism is its o^\^l measure of environmental 

 divergence. The human distinction between artificial 

 building and natural cliff does not necessarily imply 

 equal discrimination on the part of the House -Martin ; 

 and if to the latter there is no distinction, then there 

 has been no true adaptation, since adaptation implies 

 structural or functional re-adjustment. 



The species frequenting cultivation fall into two groups 

 — those which both feed and nest within its limits, and 

 those which feed in the area but do not remain to rear 

 their young therein. It seems clear that, broadly speaking, 

 the barrier to full colonization in the second group has 

 been essentially or primarily a lack of nesting-groimd. 

 If the point is obvious, it is yet not unimportant. 



Of the progress and sequence of adaptation — adjustment 

 to new environmental conditions, as distinguished from 

 simple range -extension — little is known, since it is racial 

 rather than individual, dependent upon the cumulative 

 experience of successive generations.* It will vary 

 according to species. Its possibility and progress may be 

 expected to be in direct proportion to the degree of 

 resemblance between old and new environments. A 

 species Avill show power of adaptation to new conditions 

 in inverse ratio to the perfection of its adjustment to 

 the old — ^^-hich latter is presumably, other things equal, 

 a reflection of the duration of racial experience. In 

 other words, the comparative plasticity of the generalized 

 type (mental as physical) is conducive to relatively rapid 

 re-adjustment. 



The varying fixity and stability of the constitutional 

 and mental characters which Irak a species to its 

 environment suggest that the process of adaptation may 

 be uneven. The complex named behaviour represents 

 or includes a series of specific and generalized " instincts " 

 and hereditary dispositions or tendencies — ^the former 

 relatively pure and inflexible, showing the extreme of 

 inheritance-intensity ; the latter relatively variable and 

 elastic, modifiable according^. Between the one extreme 



* Or, if this be objected to as too metaphorical, it may be otherwise 

 put that racial environmental experience is a necessary nurse of 

 adaptive congenital variations ; and the perfection of the adjustment 

 will be related to the endurance of such racial experience. 



