FALCONS. 53 



inferior tract is always divided, and thus double, in one half of its extent in these birds, they 

 cannot for this reason be confounded with the true Vultures, and it would really be only to 

 Gypaetos and Neophron that the two anomalous Falcons above referred to would nearly approach 

 in the formation of the inferior tract. From the Vultures of the New World, on the other hand, 

 the Falcons are quite sufficiently distinguished by the absence of the insular space in the pectoral 

 tract, and the complete division of the inferior tract on the neck. 



However, the pectoral portion of the inferior tract is always very variable throughout the 

 Falcons ; and besides the complete coalescence of the branch with the main stem, and its perfect 

 separation therefrom, there occurs a complete series of intermediate steps, which show how the 

 one form gradually passes into the other. Thus the branch is often free only at the end (F. 

 bidentatus TEMM. PL Col., 38), in other cases as far as the middle (F. lopJtotes TEMM., PL Col., 

 10), or to within one third from the base (Gypogeranus serpentarius), or, lastly, quite free, as I have 

 figured it in Fakofulvus (PI. II, fig. 3). The latter case is the most frequent. 



Not less variable, and consequently equally inapplicable as a characteristic element, is the 

 posterior part of the spinal tract. In the Vultures it presented us with a character for the pterylo- 

 graphic distinction of Gyps and jfigypius, in its two different forms ; and its form in the Falcons is 

 still more liable to change. The narrow, band-like form, such as occurs in Gypaiitos and 

 jEyypius, appears to be most generally diffused ; but it is by no means a constant character, even 

 in those species in which it occurs, as the posterior part is sometimes remarkably short, as in 

 Falco buleo, and sometimes long and reaching to the fork of the anterior part, as in F. milvus ; 

 but even then it usually exhibits that division into the divergent rows of feathers (Plate II, 

 fig. 4) which also occurs in the above-mentioned Vultures. Another more constant, and there- 

 fore more characteristic form, consists in the complete division and dilatation of the two limbs of 

 the tract as far as the caudal pit, which characterises the genera Rhynchodon (Plate II, fig. 6) 

 and Pandion. More rarely I have met with a third primary form, which occurs as a generally 

 dilated tract, sparsely feathered at the sides (PI. II, fig. 5). Intermediate between this form and the 

 first-mentioned are two American Falcons (F. melanops TESIM. PL Col., 105, and F. anthracinus 

 LICHT., from Mexico), inasmuch as in these there is a narrow dorsal band of uniform breadth, but 

 inside it some sparsely scattered contour-feathers. 



In the enumeration of such differences I must not omit to mention that the lumbar tract 

 also falls into the category of variable pterylographic structures, being sometimes entirely wanting 

 and sometimes present, but never, apparently, attaining any considerable extension or strength. 

 The occurrence of powder-down tracts in its vicinity has already (p. 37) been mentioned 

 generally, and will occupy us again hereafter. 



I shall say nothing at present of the variability of structure of the wings and tail, as this is 

 well known, and has already been generally employed by ornithologists for characterising their 

 numerous subgenera. I need only repeat here that all the Falcons possess ten primaries and 

 twelve tail-feathers. 



If we now take into consideration the differences of the pterylosis just indicated, in order to 

 arrange the Falcons in accordance with them, it is a matter of course that this classification 

 cannot be a natural one, being only drawn from a single character. We must, therefore, not 

 expect this from it, but rather rest satisfied with the result here given, namely, finding the 

 pterylographic multifariousness summarily represented in it. This, indeed, was our problem. 



