NATURAL SELECTION: II 493 



other hand, evidence accumulates that females detect small irregularities 

 in the courtship behavior and that even seemingly minor irregularities may 

 be sufficient to cause a female to refuse a male. As mentioned earlier (p. 

 472), such refusal appears to form a means by which interbreeding be- 

 tween closely related species is prevented. Thus differences in courtship 

 ritual form one of the means of reproductive isolation. We mention the 

 matter here because it has bearing on one a priori objection sometimes 

 raised to the theory of sexual selection: that the theory presupposes that 

 females possess a discriminating power which we can hardly grant their 

 having, particularly if they are insects. Evidence cited earlier indicates 

 that female insects have surprising powers of discrimination. 



Returning to such matters as the bright colors and songs of male birds, 

 we note that investigators have found that many of these features do not 

 have as their principal function the pleasing of females. Some bright 

 colors are warning or threatening devices. Observation and experiment on 

 the common robin of England have demonstrated that the red breast, con- 

 spicuously displayed during the breeding season, is a device for threatening 

 other males which might seek to invade the territory the individual has 

 claimed for himself. The same is true of conspicuous plumages of other 

 birds that establish home territories. The songs of male birds serve the 

 same purpose. Song has "its prime function as a 'distance threat' to rival 

 males and its secondary function as an advertisement, so long as the singer 

 is unmated, to unmated females" (Huxley, 1942). There is nothing to pre- 

 vent, of course, bright colors from serving both as warnings to rivals and as 

 lures for possible mates. We readily appreciate that if the establishment 

 of home territories is advantageous to the species, in terms of welfare of 

 the young, characteristics which aid in the establishment and maintenance 

 of the territorial system will be furthered by natural selection. 



Some of the conspicuous characteristics ascribed at times to sexual selec- 

 tion may serve a variety of functions in the lives of their possessors. Some 

 of the functions are : ( 1 ) recognition characters, means by which a female 

 recognizes a male of her own species; (2) warnings to rivals or to other 

 animals that the possessor is dangerous; (3) mimicry, either Batesian or 

 Mijllerian (pp. 370-373). All such categories, and others we have not 

 mentioned, fall within the province of natural selection since they affect 

 either the survival or reproductive success of their possessors. In fact, we 

 may expect that natural selection will tend to favor any factor or character- 

 istic that increases effectiveness in reproduction. Another such factor con- 

 sists of the means of stimulating females to reproductive activity. In birds, 

 especially, mating and reproduction are largely under control of the higher 



