XV 
THE RELATION OF FACTS TO THEORIES 
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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. galeritus 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 
