254 BRITISH BIRDS. 
disappear in the adult of C. leucogaster, but which ware Ree 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 q 
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 Palearctic 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 Palearctic 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 C. 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 Palearctic 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 5 
they may possibly be due to undisturbed climatic influence. In th e 
British Islands C. aguaticus 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 Py 
nees C. aquaticus-albicollis occurs, an intermediate form between the 
British and Mediterranean races. The latter, C. albdicollis, differs in havin 
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