NATURAL SELECTION; SEXUAL SELECTION 79 



life, ahvavs so obscured bv dirt and filth that there can be no 



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question of display to the female eye about them. The dancing 

 swarms of many kinds of insects are found to be composed of 

 males alone w r ith no females near enough to see; it is no case of 

 an excitatory flitting and whirling of many males before the eyes 

 of the impressionable females. Of many male katydids singing 

 in the shrubbery will not for any female that particular song be 

 loudest and most convincing that proceeds from the nearest 

 male, not the most expert or the strongest stridulator? Simi- 

 larly with the flitting male fireflies; will not the strongest gleam 

 be, for any female, that from the male which happens to fly 

 nearest her, and not from the distant male with ever so much 

 better, stronger light? Even in the human species, propin- 

 quity is recognized as the strongest factor in the choice of mates. 



Several other serious objections can also be urged against 

 the sexual selection theory, but the most important one of them 

 all is that all the evidence (though it is little in quantity as 

 yet, although of good quality) based on actual experiment, is 

 strongly opposed to the validity of the assumption that the 

 females make a choice among the males based on the presence 

 in the males of ornament or attractive colors, pattern, or special 

 structures. Such experiments have been undertaken by Diiri- 

 gen and Douglas w r ith lizards, and by Mayer with moths. 



It must be said, however, in closing this brief discussion 

 of the sexual selection theory, that no replacing or substitute 

 theory of anything like the same plausibility has yet been 

 offered to take its place. 



There is no question that, in many cases, brilliancy of 

 breeding colors, development of processes, and the like, is 

 often correlated with superior vigor. This is especially true 

 among fishes and birds. This reason could, however, not at 

 all account for such structures as the highly specialized stridu- 

 lating organs of certain insects. The problem of the secondary 

 sexual characters, especially of those which seem to stand in 

 opposition to the natural selection theory, is one of the most 

 pressing in present-day biology. 



