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 aff'ect that system, they must affect 

 also the internal parts with which 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 c,iute 

 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 similai-ity is, as it seems but fair to suppose, due to their existence 

 under similar external conditions. But the claws 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 not 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 were made of it; but if 

 deprived of claw and hoof, or an exchange made of them, very chfferent 

 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. 



VOL. I. 2 E 



