Vol. XXIII 



xxiin en 



i906 Clakk, Pterylosis of Sivifts and Hummingbirds. oy 



tions, still open to discussion, which may be stated as follows: 

 Is the pterylosis of a swift sufficiently similar to that of a humming- 

 bird to give support to the view that they have a common ancestry? 

 If so, is that of the Caprimulgi sufficiently similar to warrant the 

 belief that they also have the same ancestry? There are some 

 other interesting questions which arise in connection with these, 

 particularly as to whether the swifts and swallows have anything 

 in common and as to what birds are most nearly allied to the 

 groups considered in this paper. But lack of sufficient reliable 

 evidence forbids any attempt to touch on such questions. Let 

 us turn to those first stated: 1. Is the pterylosis of a swift suf- 

 ficiently similar to that of a hummingbird to give support to the 

 view that they have a common ancestry f 



In the judgment of the writer, yes. What may properly be 

 called the fundamental plan of the pterylosis is essentially the 

 same. That is to say, each group has apparently sprung from 

 an ancestor in which the head was fairly uniformly, though sparsely 

 feathered except for an apterium over the eye; the neck had the 

 upper and lower cervical tracts united anteriorly, so that there 

 was no lateral cervical apterium; there was a conspicuous nuchal 

 apterium; the back was well covered by a broad dorsal tract con- 

 taining an'elongated spinal apterium; but not fully distinct from 

 the femoral tracts or even from the numerals; the lower cervical 

 tract was deeply forked and continuous with the conspicuous 

 sternal tracts; and the latter were separate from each other, and 

 extended on to the belly as short, wide, ventral tracts. As regards 

 the dorsal tract, it seems to me probable that this ancestral form 

 was a bird in which the humeral, dorsal, and femoral tracts were 

 more or less connected, and the two latter at least not sharply 

 defined, somewhat as Nitzsch figures the dorsal surface of Colius; 

 this condition was followed by such a pterylosis as Nitzsch shows 

 for Cuculus, in which the spinal apterium has appeared; from 

 this arrangement the dorsal tract of the hummingbird would arise 

 with little change, while the cypseline condition would follow 

 further condensation of the dorsal tract, accompanied by develop- 

 ment of the femorals. In addition to this agreement in funda- 

 mental plan, the swifts and hummingbirds have so many details, 

 of more or less importance, in common, we may say further of 

 their ancestral form that it had 10 primaries with the 3 outermost 



