316 ME. J. L. BONHOTB ON THE [June 18, 



103. NUMIDA PTILORHYKCHA. 



Numida ptilorhyncha Licht. ; Grrant, Cat. B. Brit. Mus. xxii. 

 p. 379 (1893) ; Peel, t. c. p. 330. 



No. 58. S ad. Biji (1200 ft.), Jan. 12, 1899. 



4, On the Evolution of Pattern in Feathers. 

 By J. L. BoNHOTE, M.A., F.Z.S. 



[Eeceived May 30, 1901.] 

 (Plates XIX. & XX.^) 



The pattern on the feathers of birds is a subject deserving of a 

 somewhat more careful study than has been hitherto accorded to it ; 

 my attention was first drawn to the subject by having at different 

 times observed several notable varieties of young Sparrow-hawks. 

 I have since studied the very large series of that species in the 

 National Collection. The feathers on the breast of the typical young 

 Sparrow-hawk are shown in diagrammatic representation in the 

 drawing (Plate XIX. fig. 1). The ground-colour of the feather is 

 white, while along the proximal edge of each of the transverse bars, 

 which ai-e dark brown, there is a light margin of yellowish, as well 

 as in the centre of the terminal spot. Such, then, are the markings 

 of the typical breast-feather of a young Sparrow-hawk; but, from a 

 study of a large series of these birds, endless varieties of this pattern 

 may be noticed, showing clearly the gradual evolution of the barred 

 markings from a simple longitudinal streak, and giving as, as I hope 

 to show, a clue to the patterns on the feathers of all birds, or rather 

 on all the birds treated of in this article, and also giving us a hint as 

 to what extent of evolution they have undergone. A reference to 

 the figures will enable the evolution of one pattern from another to 

 be more clearly understood. The figures (see Plate XIX.), although 

 diagrammatic, are accurate representations of actual feathers of 

 Accipiter nisus. In fig. 2 ^ we may note a simple darkening of the 

 rhachis, but with the colour more intense at certain places. The 

 next stage may be noticed in fig. 3, where the rhachis is pure 

 white in between thicker blotches of the darker colour. Figs. 4, 5 

 show further stages in the gradual forming of a bar. 



Another mode of attaining the same result is shown in fig. 6, 

 where the longitudinal stripehas not become broken, but has merely 

 spread out into bars still connected by a darker portion along the 

 rhachis. Fig. 7 is another stage of this method in which the 

 proximal bar has become cut off and distinct, while the terminal 

 spot is much larger and has a great tendency to reach the bars 

 above it. From this to fig. 8 is an easy stage in which the horns 

 have grown up rather more and are farther removed from the edge 

 of the feather, forming a heart-shaped marking. It may be noticed, 

 by the way, that this form most nearly approaches the typical 

 feather fig. 1 ; the only difference being that in fig. 1 the space 



^ For explanation of the Plates, see p. 326. 



^ Where no Plate is mentioned the figures refer to Plate XIX. 



