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154 NATURAL SELECTION vnalso in the forms given to living organisms"(Reign of Law,p. 248).Here the statement that" no connection can be conceivedbetween the splendour of the humming birds and any functionessential to their life," is met by the fact that Mr. Darwinhas not only conceived but has shown, both by observationand reasoning, how beauty of colour and form may have adirect influence on the most important of all the functions oflife, that of reproduction. In the variations to which birdsare subject, any more brilliant colour than usual is believedto be attractive to the females, and would therefore lead tothe individuals so adorned leaving more than the averagenumber of offspring. There are some indications that this kindof sexual selection does actually take place, and the laws ofinheritance would necessarily lead to the further developmentof any individual peculiarity that was attractive, and thus thesplendour of the humming birds is directly connected withtheir very existence. It is true that "a crest of topaz maybe no better than a crest of sapphire," but either of these maybe much better than no crest at all ;and the different conditionsunder which the parent form must have existed indifferent parts of its range will have determined different1variations of tint, either of which were advantageous. Thereason why female birds are not adorned with equally brilliantplumes is sufficiently clear ; they would be injurious by renderingtheir possessors too conspicuous during incubation.Survival of the fittest has therefore favoured the developmentof those dark green tints on the upper surface of so manyfemale humming birds, which are most conducive to theirprotection while the important functions of hatching andrearing the young are being carried on. Keeping in mindthe laws of multiplication, variation, and survival of thefittest, which are for ever in action, these varied develop->1Since writing this essay I have come to the conclusion that merediversity of colouring between species is an important factor in their differentiation,serving as a means of recognition, and thus preventing crossunions.See Damnnism, p. 217. I have also been led to doubt the realityof the fact of female selection of slight differences of colour on which Mr.Darwin relied, but it has not been thought advisable to alter the passageswhich seem to admit it, as they represent my belief at the time they were