32 BRITISH BIRDS. [vol. viii. 



Theoretically, extension may be due to such features 

 as numerical increase ; or to environmental change, or 

 to removal of a barrier (objective or subjective) hitherto 

 preventing access to the new areas ; but the immediate 

 point is less the origin, than the method and progress, 

 of range-extension. In spreading, a species seeks prefer- 

 ably its natural habitat ; that is to say, the colonization 

 of new areas ecologically coiacident with those of origin 

 tends to precede the colonization of new environments ; 

 and the rate of increase and spread will depend less 

 directly upon fecundity than upon the extension of the 

 necessary environmental conditions. In this country the 

 Tufted Duck has shown a tendency to increase more 

 rapidly, and to spread more widely, than the Pochard ; 

 but the discrepancy between the species is probably more 

 nearly related to an miequal supply of their respective 

 ecological requirements, than to unequal fecundity. 

 Environment guides and controls, even A\here it does 

 not originate, increase. 



Into the primitive plant and faunal associations of this 

 country, the activities of man, as exemplified in forest 

 destruction and cultivation, have driven an ever- widening 

 wedge of " artificial " conditions. Notwithstanding the 

 expulsion of the original life-forms involved in the 

 destruction of their environmental requisites, the succeeding 

 artificial areas have not remained barren of life. In 

 proportion as they have extended, the vanished fauna has 

 been succeeded (replaced, not expelled) by new groups, 

 whose constituents have been independently derived from 

 the surrounding and retreating primitive faima. But 

 adjustment to the new conditions by the colonizing 

 species is more apparent than real, in that such areas 

 have been populated less immediately by the more 

 adaptive species than by those whose original and 

 natural environment chanced to be most closely simulated 

 by the work of man. Man-altered ground has attracted, 

 as nesting-species, the Partridge rather than the Grouse, 

 the Com-Crake than the Water-Rail, the Lapwing than 

 the Golden Plover, the Swallow and House-Martin rather 

 than the Sand-Martin. The colonization of " artificial " 

 ground in fact appears to illustrate rather simple range- 

 extension than adaptation, inasmuch as the ecological 

 conditions approximate to those of the original centres 

 of the colonizing species. It is this relation which has 

 been essentially the determining factor in extension to 

 man-made territory. It will not be claimed that the 



