EVOLUTION OF THE COLORS OF BIRDS. 209 



female, or how they may even be wholly lacking in the 

 latter. The female selects the mate and not the male; 

 and it would be quite possible for natural selection so to 

 adjust matters that the female would always prefer to 

 mate with a male marked in one manner. Those females 

 which selected for mates individuals differently marked 

 {i.e., belonged to a different race, and consequently 

 sterile when crossed), would leave no offspring and the 

 tendency would accordingly be for those only to survive 

 which selected mates of the one pattern. In this man- 

 ner by a process of natural selection a form of sexual 

 selection having no relation whatever to aesthetic 

 choice, but simply to the survival of the fittest would be 

 instituted. Wherever a marked character is present in 

 the male which seems to be more of a mark of recog- 

 nition than of beauty, and absent in the female, it may 

 be explained, it seems to me, according to this principle 

 which may be called the Law of Sexual Recognition. 

 The crown patches? of the white-crowned sparrow (Zono- 

 trlchia leucophrys), and the golden-crowned sparrow (Z. 

 coronata), the black cap of the male gnatcatcher (Poli- 

 optila), the black throat patch of the male house-sparrow 

 (Passer dotnesticus), and many other instances, are exam- 

 ples of the workings of this law. This law of sexual recog- 

 nition might frequently be coupled with the law of sexual 

 intensification, so that both sexes would progress along 

 a certain line, the males in advance, until the latter had 

 reached the goal of specialization, after which the female 

 would ''catchup" to the male. Markings which had 

 thus been sexual recognition marks would become ordi- 

 nary discriminative marks and finally directive marks. 

 These sexual recognition marks are closely connected 

 with the fourth and last class, viz.: socialistic markings. 

 These are such marks as assist in the domestic and 

 social relations of the family. They are particularly 

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