1866.] PROF. W. LILLJEBOItG ON THE CLASS OF BIRDS. 7 



Irritability seems to us to be the most distinguishing character for 

 birds ; and this should consequently be taken into consideration 

 more than others with regard to tbeir classification. The swimmers 

 seem to us the lowest, from their showing a tendency to the lowest 

 form of vertebrated animals — the fish-form. In the Aptenodytidce, 

 where the wings resemble fins, and where they, as in all other diving 

 birds, serve as such, we have this form most strongly designated. 

 The heavy, clumsy structure, with small wings and short legs, also 

 makes them generally less active than other birds, and shows a lower 

 development of the type of bird. This, however, is not the case with 

 all the swimmers ; and the order Longipennes gives us instances 

 where swimmers possess a high degree of activity. 



The Passerine birds (Passeres) seem to us to possess the highest 

 irritability, and to be those in which the nature of birds has reached 

 its highest development. We do not by irritability mean the mus- 

 cular strength alone, but vivacity and activity generally. Where this 

 is most manifold, most changing and constant, it is the most developed. 

 We find in the Passeres " the power to stay and move with ease as 

 well on the ground as in the trees or in the air, and to make their 

 presence known by characteristic melodious notes" (Sundevall) ; we 

 find them in a constant and manifold motion, and they let us con- 

 stantly hear their notes either as song or as affectionate voices. The 

 birds of prey have generally been placed highest, and been con- 

 sidered the most developed, in consequence of their muscular strength 

 and strong flight, and their thereby supposed high degree of irrita- 

 bility ; but by keeping them in captivity we find at once that the birds 

 of prey are dull birds, and that they, as regards irritability, are far 

 behind the Passeres. They remain for a long time silent and quiet, 

 and do not generally show any activity, unless they are frightened 

 or driven by appetite for food. The Passerine birds, on the con- 

 trary, are in captivity constantly in motion, and let us incessantly 

 hear their lively song and affectionate voices. Besides we cannot in 

 a system place the birds of prey far from the lower groups, of the 

 Columbine and the Gallinaceous sections, without violating natural 

 affinities based upon important characters. They correspond with 

 these lower groups as regards external characters in the nature of 

 their wing-coverts, and, as regards interior anatomical characters, iu 

 the nature of their carotides communes. Some of them, for instance 

 those of the Vulturine section, exhibit, with regard to their form, a 

 near analogy with some of those of the two mentioned groups. We 

 may, for instance, compare a Condor with a Turkey. A system that 

 places the dirty Vultures highest, does not seem to us to indicate a 

 correct idea of the nature of the birds. 



If we do not regard flight, which is common to almost all birds, 

 but consider birds with regard to the various other ways of motion 

 for which they especially are shaped, and for which their structure 

 is also adapted, we find easily that these in general may be compre- 

 hended in three different modes, viz. : 1st, swimming on the water ; 

 2nd, running on the ground ; and 3rd, climbing and jumping on 



