XV THE RELATION OF FACTS TO THEORIES 307 



differ considerably from one another. The sexual 

 differences are especially well marked in the latter, 

 in which the male is of a bright brown-red colour, a 

 tint comparatively rare among the humming-birds. 

 The female exhibits colours of a more usual type, 

 but is remarkable in possessing a brilliant metallic 

 crest, an ornament which is very rare among female 

 humming-birds. The male of the other species is 

 very like the female of E. fernandensis, except that 

 his crest is red instead of green, while his own female 

 is very plain, and without a crest. Now, if Mr. Wallace 

 admits that the brilliancy of the male E. fernandensis, 

 as compared with his mate, is due to his greater 

 vigour and vitality, surely it is not unreasonable to 

 conclude that the general greater brilliancy of this 

 species, as compared with E. galeritus, is due to its 

 greater vigour and vitality. In other words, bearing 

 in mind that the male of E. gakritus is hardly more 

 brightly coloured than the female of E. fernandensis, 

 may we not say that the two species bear to each 

 other, as regards vitality, the same relation as the 

 male and female of E. fernandensis bear to one 

 another ? If this be granted, then surely, other things 

 being equal, the coloration of a species bears some 

 relation to its vitality — that is, it is primarily deter- 

 mined by the physiological condition of the organism 

 and not forced upon it by the stress of environmental 

 conditions (see the Evolution of Sex). Again, if this 

 be so, much of the elaborate treatment of colour 

 phenomena in the early part of Mr. Wallace's book 

 seems needless. If we may account for the colours 

 of many birds as the _^incidental consequences of 

 physiological conditions, then surely we need no 



