CHAP. XIII.] 
THE AUSTRALIAN REGION. 
395 
peculiar to the Australian region. Another of the universally dis- 
tributed families which have their metropolis here, is that of the 
Columbidae or pigeons. Three-fourths of the genera have represen- 
tatives in the Australian region, while two-fifths of the whole are 
confined to it ; and it possesses as many species of pigeons as any 
other two regions combined. It also possesses the most remark- 
able forms, as exemplified in the great crowned pigeons (Goura) 
and the hook-billed Didunculus, while the green fruit-pigeons 
( Ptilopus ) are sometimes adorned with colours vying with those 
of the gayest parrots or chatterers. This enormous development 
of a family of birds so defenceless as the pigeons, whose rude 
nests expose their eggs and helpless young to continual danger, 
may perhaps be correlated, as I have suggested elsewhere (Ibis, 
1865 , p. 366 ), with the entire absence of monkeys, cats, lemurs, 
weasels, civets and other arboreal mammals, which prey on eggs 
and young birds. The very prevalent green colour of the upper 
part of their plumage, may be due to the need of concealment 
from their only enemies, — birds of prey ; and this is rendered 
more probable by the fact that it is among the pigeons of the 
small islands of the Pacific (where hawks and their allies are ex- 
ceedingly scarce) that we alone meet with species whose entire 
plumage is a rich and conspicuous yellow. Where the need of 
concealment is least, the brilliancy of colour has attained its 
maximum. We may therefore look upon the genus Ptilopus, 
with its fifty species whose typical coloration is green, with 
patches of bright blue, red, or yellow on the head and breast, 
as a special development suited to the tropical portion of the 
Australian region, to which it is almost wholly confined. 
It will be seen from the sketch just given, that the ornitho- 
logical features of the Australian region are almost as remark- 
able as those presented by its Mammalian fauna ; and from the 
fuller development attained by the aerial class of birds, much 
more varied and interesting. None of the other regions of. the 
earth can offer us so many families with special points of 
interest in structure, or habits, or general relations. The 
paradise-birds, the honeysuckers, the brush-tongued paroquets, 
the mound-builders, and the cassowaries — all strictly peculiar 
