1896. ] METALLIC COLOURS OF BIRDS. 287 
gradual to show that the stalk of the metallic barbule is the 
rudimentary lamina, and the club-shaped body a modification of the 
distal region of an ordinary barbule. As was observed by Gadow, 
the cilia are totally suppressed. 
To contrast with this feather, we may take one from the bril- 
liant green gorget of the Humming-bird Basilinna leucotis (Pl. XI. 
fig. 7). Such a feather is comparatively short, and the especial bril- 
liancy is confined to a darkly pigmented apical band. The apex of the 
feather is very abruptly rounded and the barbs are closely connected 
so as to give a plate-like or scaly appearance. The surface is 
strongly marked with furrows, which when examined with a lens 
are seen to correspond to the barbs, each of which lies as it were 
atthe bottom of a trough. The sides of this trough are formed by 
the two rows of barbules, which are inserted at a distinct angle. 
The sides of the trough have an unequal slope, as the proximal 
barbules are inserted at a larger angle than-the distal. A further 
point of interest is that the naked barbs are prolonged beyond the 
apex of the feather, producing the appearance of a very delicate 
fringe. This prolongation of the naked barb was noticed many 
years ago (see the Introduction to Gould’s Monograph of the 
Humming-birds), and its meaning will be explained later on. 
Examined microscopically, the downy barbules of this feather 
show nothing particularly worthy of notice. The remaining bar- 
bules have alla well-developed lamina, and a distal region usually 
well provided with cilia. Those nearest the apex of the barb, that 
is those which are very brilliantly metallic, are very darkly pig- 
mented (Pl. XI. fig. 9). Those near the base of the upper barbs are 
much less strongly pigmented, and show very little metallic colour. 
Between the two extremes there are also other marked differences 
(Pl. XI. figs. 8 and 9). Thus in the basal barbules there is no marked 
angle between the proximal flattened region and the distal cilia- 
bearing region. In the apical barbules the proximal region is widened 
and has a well-marked infolding, while the distal region is inclined 
to it at such an angle as to be almost invisible in surface view 
(Pl. XI. fig. 10). 
Microscopically, the naked tip of the barb is seen to be furnished 
with rudiments of barbules. 
As this type of metallic feather occurs in all the Humming- 
birds of which the feathers were examined, it is obvious that Gadow’s 
statement that all metallic barbules are devoid of cilia is too 
universal. It is certainly true for a great number of cases (Sun- 
birds, Peacock, &c.), because in these it is the region which ordi- 
narily bears the cilia—the distal or filamentous region—which is 
modified into the colour-producing structure, and in the course of 
the modification the cilia are lost. In the Humming-birds, on the 
other hand, it is the proximal region of the barbule which is the 
colour-producing structure, and in consequence the filamentous 
region is not affected and may bear cilia as usual. In the feather 
described above, the cilia are exceedingly well-developed in the 
metallic barbules. 
Though this paper is not concerned with the physical cause of 
