1896.] METALLIC COLOUES OF BIRDS. 293 



in the wing-quills iu the direction of diminished efficiency for flight 

 would be checked by natural selection. 



So far we have seen how the metallic colours both of Sun-birds 

 and Humming-birds depend in each case upon a combination of a 

 certain structure and a black pigment. In conclusion something 

 may be said as to the colours themselves. In the Sun-birds a 

 greenish-blue seems to be the most primitive metallic colour, and 

 this is a very common tint elsewhere, e. g. Peacock, Quezal, &c. 

 According to Gadovv, one of the reasons why any metallic feather 

 does not display all the colours of the spectrum is probably because 

 the overlapping of successive colour-producing structures cuts out 

 certain of the rays. If this overlapping really occurs it seems not 

 unnatural to conclude that the middle rays of the spectrum, those 

 in the neighbourhood of the green, would be least likely to be 

 affected, and we would thus get green as a primitive metallic 

 colour. The combination of this structure with a surface sculptur- 

 ing might produce a purple or violet tint ; the absence of red and 

 yellow may not improbably be a result of physical conditions. 

 Walter explains the rarity of red and yellow metallic colours as 

 due to the nature of the pigments contained in the coloured tissues, 

 but this again is difficult to harmonize with our knowledge of such 

 pigments. 



There can be little doubt that in Humming-birds a greenish- 

 yellow is the most primitive metallic tint. It is suggested even in 

 the " hermit " foru)s, and is very widely spread on contour-feathers 

 elsewhere. In the absence, however, of any suggested physical 

 explanation of the metallic colours of Humming-birds, it would 

 perhaps be premature to attempt to account for the wonderful 

 range of colour found in the family. As to the distribution of 

 metallic colour one or two facts still remain to be noticed. Thus 

 metallic colour is not always characteristic of the male. In Euste- 

 phanus fernandensis female metallic colour is more or less distinctly 

 present over nearly the whole of the upper and under surfaces, 

 the tail-quills show bright metallic colour, and the head bears a 

 special metallic crest. The male, on the other hand, has no metallic 

 colour except the bright crest, the rest of the body is cinnamon- 

 coloured and without metallic gloss. The absence of metallic 

 colour is apparently to be accounted for here by the absence of the 

 usual blackish-brown pigment. Again, a specimen marked Topaza 

 pella, young male, which was examined, showed metallic-greenish 

 feathers in the upper part of the head, a spot which in the adult 

 is covered with black feathers. Similarly, Salvin notices that in 

 Lampornis mango the throat in the young bird is covered with 

 glittering green feathers, and in tlie adult with pure black ones. 

 Thus apparently an excess of black pigment is as fatal to the 

 display of metallic colour as its total absence. As to the relation 

 between a black colour and metallic tints there are some other 

 interesting facts. In Cyanolesbia gorgo the tail-quills are greatly 

 elongated and show gorgeous metallic colour, but this is confined 

 to the distal end of the feathers, the proximal region being a 



