Finches Graduated Beaks
The most curious fact is the perfect gradation in the size of the
beaks in the different species of Geospiza, from one as large as
that of a hawfinch to that of a chaffinch, and (if Mr. Gould is
right in including his sub-group, Certhidea, in the main group)
even to that of a warbler. The largest beak in the genus Geospiza
is shown in (Plate 81) Figure 1, and the smallest in Figure 3; but
instead of there being only one intermediate species, with a beak
of the size shown in Figure 2, there are no less than six species
with insensibly graduated beaks. The beak of the sub-group
Certhidea, is shown in Figure 4. The beak of Cactornis is somewhat
like that of a starling, and that of the fourth sub-group,
Camarhynchus, is slightly parrot-shaped. Seeing this gradation and
diversity of structure in one small, intimately related group of
birds, one might really fancy that from an original paucity of
birds in this archipelago, one species had been taken and modified
for different ends. In a like manner it might be fancied that a
bird, originally a buzzard, had been induced here to undertake the
office of the carrion-feeding Polybori of the American continent.