VOL. VIII.] RELATIONS OF BIRU-DISTRIBUTION. 41 



parallel faimal contrasts ; nor do uniform climatic con- 

 ditions necessarily involve faunal coincidence. Within 

 British limits, environment is more effective than climate. 



The strength of the connexion between the vegetation 

 and the dependent avifauna is otherwise made clear in the 

 close correlation between plant-succession and bird-succes- 

 sion. Whether in the slower successions characteristic of 

 natural conditions, such as in the silting up of a lake through 

 its subsequent stages of marsh and moor ; or whether in 

 the more striking, because more rapid, examples common 

 imder artificail conditions, such as the succession involved 

 in the conversion of primitive ground to arable land, or in 

 the growth of a plantation — a closely-linked bird-succession 

 is alike an accompaniment. In the case of the plantation 

 the parallelism is complete in detail : in both plant- and 

 bird-succession the pioneer species are replaced by later 

 immigrants, which in turn disappear, or yield the dominant 

 position, to the '' clunax " species adapted to full tree-growth. 

 B}^ virtue of environmental bias and nesting-requirements, it 

 is probable that the bird-succession follows more immediately 

 the plant-succession than that of the food-complex. 



Comparable to ecological succession, but in some ways 

 contrasted, is the yearly sequence knovAH as seasonal 

 succession. It is alike in its species-succession related to 

 change in vegetation form, contrasted in its regular recurrence, 

 and in that the replacement of one species by another is less 

 characteristic. Seasonal succession in birds must be held 

 to include not only the successive immigration of migrants 

 and seasonal residents, but also the succession of stages in 

 the life-history of the species co-habiting a given locality — 

 the sequence of song, of pairing, of nesting — in short, all 

 phenomena appertaining to behaviour, since all have 

 environmental relationship. 



It might at first sight be expected that the behaviour- 

 succession in the birds of a given association is, as a succession,^ 

 in complete correlation A\ith that of the environmental 

 conditions ; and it is clear that some such relation must 

 exist. But to regard the accordance as necessarily com- 

 plete ; to assume the sequence in its present form as purely 

 an adjustment to present conditions ; is to ignore the 

 inevitable influence of the past. Theoretically, species may- 

 be expected to have reached their present environmental 

 limitations from different approaches, from varying (environ- 

 mental) distances, and at different periods in time ; and along 

 \Aith the coincidence involved in a common adjustment to 

 a common environment, it is conceivable that each species- 



