Classification of Birds 135 



Huxley indicated, and, in different animals, as Huxley 

 suggested, the exact limits of the ossification of the 

 primitive cartilages differ in extent, but occur in ho- 

 mologous situations. This primitive skull is roofed 

 over by a series of membrane bones which have no 

 connection in origin with the other portions of the 

 skull, and which have no representative in the verte- 

 bral column, but which are the direct descendants of 

 the bony scales clothing the external skin in carti- 

 laginous fishes. In one respect only was Huxley 

 erroneous. Partly b}^ inadvertence, and partly because 

 the minute details of vertebrate embryology became 

 really familiar to zoologists only after the elaborate 

 work of Balfour of Cambridge, Huxley, in his account 

 of the formation of the first beginnings of the skeleton 

 in the embryo, made confusion between the walls of 

 the primitive groove, which, in reality, give rise to the 

 nervous structures, and those embryonic tissues which 

 form the skeletal S3^stem. 



The next great piece of work which we may take as 

 typical of Huxley's contributions to vertebrate anat- 

 omy, is his classical study on the classification of birds. 

 The great group of birds contains a larger number of 

 species than is known in any other group of verte- 

 brates, and, in this vast assemblage of forms there is 

 strikingly little anatomical difference. The ostrich 

 and the humming-bird might perhaps be taken as 

 types of the extremest differences to be found, and yet, 

 although these differ in size, plumage, adaptations, 

 habits, mode of life, and almost everj^thing that can 

 separate living things, the two conform so closely to 

 the common type of bird-structure that knowledge of 

 the anatomy of one would be a sufBcient guide, down 

 to minute details, for dissection of the other. None 



