Development of Pterylosis 7 



arm, and forming what are called the scapulars; and the femoral tracts, a pair 

 of tracts over the thighs. The pterylosis of the Nighthawk is shown in the 

 accompanying figures (Figs. 3, 4). 



It has been found that the extent and distribution of the feather tracts and 

 bare spaces are relatively very uniform for certain groups of birds, so uniform, in 

 fact, that pterylosis was made the basis of an elaborate scheme of classification^ 

 of birds by Nitzsch. That it is of diagnostic value in many cases cannot be 

 denied, but when relied upon too implicitly it not infrequently leads to what are 

 obviously unnatural assemblages. Further than this we are still in ignorance of 

 the pterylography of a vast number of birds, for the subject has been largely 

 neglected since the time of Nitzsch. When a larger array of facts is at hand it 

 may be possible to extend its usefulness. 



The only birds at present known to have a continuous feather covering are the 

 Penguins and Screamers, although formerly the Ostriches and their immediate 

 allies were supposed to fall within this category. Apteria have long been known 

 to occur in the embryos of certain "Ratites," as the Ostrich, Rhea, and Apteryx, 

 but recently Pycraft has shown that small but relatively important apteria occur 

 in the adults of all members of this group. 



Development of Pterylosis. In a communication before the American 

 Ornithologists' Union, November 18, 1903, Dr. Hubert Lyman Clark presented 

 some important results of his examination into the development of the pterylosis. 

 The material studied consisted of 54 embryos belonging mainly to different 

 groups of birds such as Herons, Rails, Sparrows, Woodpeckers, Hummingbirds, 

 and Swallows. In every instance he was able to distinguish the outlines of certain 

 of the principal feather tracts before the body of the bird had assumed very 

 definite form, for example, before the shape of the head could be distinctly 

 made out. In all cases examined the caudal tract was the first to be outlined, and 

 further the middle pair of tail-feathers was uniformly the first to appear. At this 

 stage in the development of the embryo the tail was disproportionately long and 

 had the feathers disposed along it in pairs, a condition very suggestive at least 

 of the tail of Archseopteryx. As a further interesting result it was shown that 

 in the specimens studied the secondaries of the wing were the first to appear, 

 thus confirming the result of Pycraft's studies on the development of the Mound- 

 Builders (Megapodidce). The primaries were found to be developed distally; 

 that is, from the angle of the wing toward the tip. It would seem that characters 

 as deep seated as these have been shown to be must have an important bearing 

 on the taxonomy of birds when we are in possession of a sufficient body of facts 

 to permit of generalization, but, as Dr. Clark pointed out, we do not yet know 

 the complete history of the development of the pterylosis of a single species of bird. 



The causes which have led to the development of feather tracts and conse- 

 quent bare spaces are not well understood. The suggestion that it is simply 

 another example of nature's economy of material seems hardly an adequate one ; 

 the explanation advocated by Mr. F. A. Lucas, namely, that it is a case of adapta- 

 tion, being decidedly more logical. On this point Mr. Lucas says : "The pterylosis 

 of all birds is more or less adaptive, having some direct relation to their habits, 



