Mr. R. F. Tomes on White’s Thrush. 385 
this, the only recordable difference in the osteology of these 
species*. 
A comparison may now be made between the Blackbird and 
Fieldfare, two species of somewhat similar size, one resident and 
the other migratory, as before, and, in like manner, characterized 
respectively by shortish and rounded wings, and by longer and 
more pointed ones. We feel quite prepared for results similar 
to those observed in the two preceding species, and are quite 
satisfied with finding that the sternal keel of the Blackbird is 
shallow as compared with that of the Fieldfare. But here the 
* The intimate connexion which must needs exist between the external 
and internal parts of the organ of flight, involves another question of con¬ 
sideration to the scientific naturalist. I allude to the influence of external 
causes. As all the feathers of a bird, including of course those used in 
flight, are simply a development of the cutaneous system, it follows that, if 
any particular conditions of existence affect that system, they must affect 
also the internal parts with w r hich it is immediately connected. However, 
we have yet to learn in what manner the plumage of birds is affected by 
external causes. The clothing feathers may be liable to modification quite 
apart from those which serve for locomotion; and this supposition (for I 
only advance it as such) has its analogy in the class Mammalia. The 
different species of Tupaia have fur of precisely the same quality and 
texture, and even colour, as some of the Squirrels of the same region; and 
the similarity is, as it seems but fair to suppose, due to their existence 
under similar external conditions. But the claw r s of these animals suffer no 
equalizing modifications; and they also are a part of the cutaneous system. 
It would be highly interesting to ascertain what part of the plumage of 
birds answers exactly to those parts of the cutaneous system of mammals 
which are developed into horny appendages subservient to locomotion and 
other uses. The claw of a tiger has as exact a relation to the whole of 
the limb on which it is fixed, and acts as perfectly with it as does the hoof 
of a horse with the limb to which it is affixed; but the hair which clothes 
the limb would seem in either case nA; necessarily to hold any direct or 
immediate relationship with the type on which the limb is constructed. 
The legs of the Tiger and Horse would perform their functions equally well 
if wholly deprived of hair, or if an exchange w r ere made of it; but if 
deprived of claw and hoof, or an exchange made of them, very different 
results would accrue. If analogy be of avail in such cases, the same ought 
to hold good with birds, and we should be able to separate correctly the 
clothing plumage from that which serves for locomotion. This might 
furnish an argument for those who deem that everything necessary for the 
study of ornithology may be found in dry skin and feathers, since the 
native and special use of the limb should be understood by the study of 
these. 
2 E 
VOL. I. 
