SEXUAL SELECTION. 213 



vital energy. When poised in the air their wings are 

 invisible, owing to the rapidity of their motion, and 

 when startled they dart away with the rapidity of a 

 flash of light. Such active creatures would not be an 

 easy prey to any rapacious bird ; and if one at length 

 was captured, the morsel obtained would hardly repay 

 the labour. We may be sure, therefore, that they are 

 practically unmolested. The immense variety they 

 exhibit in structure, plumage, and colour, indicates a 

 high antiquity for the race; while their general abundance 

 in individuals shows that they are a dominant group, 

 well adapted to all the conditions of their existence. 

 Here we find everything necessary for the development 

 of colour and accessory plumes. The surplus vital 

 energy shown in their combats and excessive activity, 

 has expended itself in ever-increasing developments of 

 plumage, and greater and greater intensity of colour, 

 regulated only by the need for specific identification 

 which would be especially required in such small and 

 mobile creatures. Thus may be explained those remark- 

 able differences of colour between closely-allied species, 

 one having a crest like the topaz, while in another it 

 resembles the sapphire. The more vivid colours and 

 more developed plumage of the males, I am now inclined 

 to think may be wholly due to their greater vital energy, 

 and to those general laws which lead to such superior 

 developments even in domestic breeds ; but in some 

 cases the need of protection by the female while in- 

 cubating, to which I formerly imputed the whole phe- 

 nomenon, may have suppressed a portion of the ornament 

 which she would otherwise have attained. 



The extreme pugnacity of humming-birds has been 



