4o8 REVIEW OF BIRDS. 



Irritability seems to ug to be the most distingnishing character for birds; and 

 this should consequently be taken into consideration more than others with re- 

 gard to their 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 AjifenodT/tidce, 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 lov/er development of th.e 

 type of bird. This, however, is not the case with all the swimmers ; and the 

 order Longipennes gives us instances wheio 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 develop- 

 ment. We do not by irritability mean the muscular 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," (Sundcvall;) we find them 

 in a constant and manifold motion, and they let us constantly hear their notes 

 either as song or as afiectionate voices. The birds of prey have generally been 

 placed highest, and been considered the most developed, in consequence of their 

 muscular strength and strong flight, and their thereby supposed high degree of 

 in-itability ; 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, imless they are frightened or driven by appetite for food. 

 The Passerine birds, on the contrary, are in captivity constantly in motion, and 

 let us incessantly hear their lively song and affc'ctionate voices. Besides, we 

 ainnot 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, in 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 

 bu'ds 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 comprehended in three difierent modes, viz : 1st, shim- 

 ming on the water; 2d, running on the ground; and 3d, climbing and jumping 

 on the branches of trees.* The hinder extremities or the legs exhibit, in con- 

 formity with this, three different forms. This induces us to divide the class of 

 birds into three primary groups or sub-classes : 1, Natatores; 2, Cursores; 3, 

 Insessores. Those belonging to the third group generally move more with the 

 assistance of their wings tlian the others, except some forms of the Natatores, 

 and show generally a higher development of the bird-type. This group also 

 furnishes the greatest variety of forms. The Natatores include about 550 

 species, the Cursores 900, and the Insessores 6,900, (Bonaparte.) 



Nitzsch has, in the treatise referred to, divided the class of birds into three 

 groups : Aves advess, Aves tcrrestres, and Aves aquaticai, which in a reverse 

 order correspond with the three groups here arranged; but he differs from us in 

 including the Columbine birds among the Terrestres, and the Grallatorial birds 



"The secoDd mode appears an iutermediate link between the first and thkd. 



