42 BRITISH BIRDS. [vol. viii. 



may have retained individual minor fimctional peculiarities, 

 inheritances of a racial past. For example, in this country- 

 there is a distinct, though trifling, difference in nesting -period 

 between the Song-Thrush and Blackbhd within uniform 

 environmental conditions. Is it necessary to assume that 

 this differentiation is connected with a corresponding seasonal 

 difference in the respective food-supplies ? — or, in fact, with 

 any effective factor xmder present circumstances ? In 

 absence of any evidence of present causes of present difference 

 we must presume that the latter is related to divergence 

 (climatic ?) in the respective ancestral conditions of the 

 species. Through selection or other moulding force an 

 adjustment of the present nesting-period to the present 

 seasonal conditions will evolve ; but since adaptation is 

 purely relative, different species will have reached varying 

 adaptive stages ; and so far as this is so, it is unreasonable 

 to expect that all seasonal functional phenomena are adapted 

 m equal and absolute relativity, and run completely parallel, 

 to present environmental seasonal conditions. 



But although present fimctional adjustment to present 

 seasonal phenomena may be to some extent, and in varying 

 degrees, modified or deflected by functional survivals of 

 adjustment to past conditions, yet no doubt an explana- 

 tion of seasonal succession must be sought in the main in 

 present conditions of existence. Alike to illustrate the 

 factors determining such succession, and to explain the 

 presumably related phenomenon of variation within the 

 species, statistical comparison is desirable. The biometrical 

 method is apphcable not less to function than to structure. 



Summary. 



While the composition of a country's avifauna is, in one 

 sense, directly related to the present and former configura- 

 tion of land-masses, the form of its dispersal within the 

 area is immediately attributable to the environmental 

 control. Such being so, a systematic presentation of the 

 facts from the environmental side is essential to a full illus- 

 tration of the M-hole subject of geographical distribution. 



The origin of the envhronmental bias, A\hich is essentially 

 the immediate link attaching the species to its present 

 habitat, must be sought in the past environmental relations 

 of the race ; it is literally or metaphorically the expression 

 in the individual of the sum of racial experience. As such, 

 its present strength is related to the endurance of ancestral 

 experience plus the relative constancy or imiformity of 



