20 PTERYLOGRAPHY. 



also resembles the continuous form in some degree, and might be regarded as continuous on a 

 cursory examination. 



The differences just indicated sometimes occur on the same bird, nay, even in the same 

 tract. For there are tracts of which one part is densely, and another sparsely feathered ; and 

 others appear strong in one part, and weak in another. Some have a sharply marked margin 

 on one side, whilst on the other they pass almost imperceptibly into the neighbouring space. 

 And we also meet with tracts which are broad in one place, but very narrow in the rest of their 

 extent. 



In this way many different forms are produced. It is true, that nearly all tracts form bands 

 longer than broad, following the direction of the length of the body, but although in many birds, 

 they are continued without interruption, in others they are broken up by real or apparent gaps. 

 A real gap is a spot in the tract, which bears no feathers at all; an apparent, or false ffap, on 

 the contrary, is one which is covered by weaker contour-feathers resembling down- feathers. A 

 tract is also often merely divided longitudinally in some birds, and double in others ; or another 

 may be double which generally occurs in a simple form ; then again, two tracts which are 

 usually distinct may unite ; and lastly, the tracts may extend or contract themselves, sometimes 

 in one form, sometimes in another. In some birds, tracts may be wholly or partially wanting, 

 which are generally present. I shall have to recur to these distinctions hereafter in the de- 

 scription of the different tracts, and therefore dwell no further upon them here, from a con- 

 viction that the indications already given will show the reader that there is a great variety in 

 them. 



Although the design of the tracts is sometimes very different in similar birds, or more 

 accordant in others which have but little affinity, the attentive observer cannot fail to perceive 

 that the tracts are partly in accordance, and partly different in the different families of birds, 

 and that they by no means occupy the lowest place amongst those characters which are regarded 

 as available for the establishment of families, or higher groups of birds. This is clear, 

 from the simple fact, that there are some whole families and many genera of birds, which 

 cannot be better distinguished from other similar forms by any external character than by the 

 form and structure of their feather-tracts. As examples of this, I shall only mention the 

 Nocturnal Rapacious birds, the Passerine, Macrocliircs, Columbina?, Gattinacete, Herodii, and 

 Limicolce ; and, as a few genera, among very many: Rhynchodon NOB. (including the Noble 

 Falcons), Pandion, Hybris NOB. (Strix Jlammea, and its allies), Cindus, Ocypterus, Merops, 

 Gallula, Cuculus, Prodotes (Indicator Cuv.), Colius, Pavo, Otis, and Psophia. 



In certain families of birds, as for example, the Cuculince, Lipofflossce, and Amphiboly, 

 the design of the feather-tracts is so discordant that it is scarcely available for the formation of a 

 general character of the family ; but then we find that the genera are all the better distinguished 

 by the design of the tracts. But when a very definite family-type of the feather-tracts makes its 

 appearance, as in the Passerine, Limicolts, Lonyipennes, Stcganopodes, and Unguirostres, the 

 generic distinctions gradually disappear, although not entirely. Thus, among the Passerines, 

 some genera have the dorsal tract divided on the back, while most of them have a simple one. 

 This tract usually forms at this spot a dilatation, which in most genera is of a rhombic form, but in 

 some is elliptical, laterally rounded, or almost circular. In other genera again, the anterior part 

 of the tract in front of the dilatation is broader than the posterior part ; or the reverse is the case. 

 The inferior tracts in this family are somewhat enlarged upon the breast, or, in many genera, 



