254 BRITISH BIRDS. 



disappear in the adult of C. leucogaster, but which were probably charac- 

 teristic of its Preglacial ancestors. In the course of ages the original 

 Dipper with the spotted underparts appears to have become separated into 

 two species. In the western form circumstances seem to have favoured 

 the development of the white of the underparts, whilst in the east the 

 reverse appears to have been the case, so that during the Glacial period it 

 is probable that there were two species of Dipper a form with white under- 

 parts in the west, and one with dark brown underparts in the east. It 

 seems not improbable that at this time the Dipper was a migratory bird, 

 its small bastard primary being possibly a relic of its past powers of flight; 

 and as the Glacial period passed away, and the rapid and important deve- 

 lopment of Palsearctic birds which accompanied the semitropical period 

 which followed took place, the Dippers seem to have caught the general 

 spirit of enterprise, and some of the eastern race seem to have spread along 

 the eastern coast of Asia and to have crossed Behring's Straits into America, 

 and, following the Rocky Mountains to Central America, seem to have 

 reached the Northern Andes. Amidst their new surroundings they have 

 comparatively rapidly changed their character ; and those birds which 

 reached South America have reverted to the particoloured plumage of 

 their ancestors, though in somewhat new and modified forms. 



As the other species of Dipper spread eastwards, the influence of the 

 changed climate, or some process of natural selection which may some day 

 be discovered, caused the underparts below the breast to become a sooty 

 black, a character which is still retained by the adults of many of the present 

 Western Palsearctic Dippers and by the birds of the year of all the Euro- 

 pean species. This circumstance has given rise to much confusion in the 

 accounts of the geographical distribution of the dark-bellied form C. mela- 

 nogaster. Dresser records it from Ireland and England, Newton from 

 Spain, and Salvin from Asia Minor. In all these cases it will probably be 

 found that the examples which have been identified as (7. melanogaster are 

 birds of the year of the species inhabiting the countries where they were 

 severally obtained. 



It seems to me that there is only one species of Palsearctic Dipper, which 

 may be divided into many subspecies or local races which are imperfectly 

 segregated, and interbreed whenever they come together. It is difficult to 

 see how any differences which they present can have any protective value ; 

 they may possibly be due to undisturbed climatic influence. In the 

 British Islands C. aquations occurs, the damp climate caused by the Gulf- 

 stream having developed the chestnut on the belly to its greatest extent, 

 and the cold having in some mysterious way blackened the brown of the 

 head and nape. Further south in the Vosges Mountains and in the Pyre- 

 nees C. aquaticus-albicollis occurs, an intermediate form between the 

 British and Mediterranean races. The latter, C, albicollis, differs in having 



