124 DARWINISM TO-DAY. 



tions of secondary sexual characters which various biol- 

 ogists have offered. These explanations will be presented 

 Alternative ex- in some detail in chapter xi, which is devoted 

 ondarysexnlr " to an exposition of the various alternative 

 characters. theories proposed to replace or partially to re- 

 place the Darwinian theories. It must be sufficient to say here 

 that the theories proposed to account for secondary sexual 

 characters mostly rest on one or both of two principal basic 

 assumptions ; first, that the secondary sexual characters are 

 produced as the result of the immediate stimulus (naturally 

 different) of the sexually differing primary reproductive 

 organs, this stimulus being usually considered to result 

 from an internal secretion of the genital organs acting on 

 certain tissues of the organism ; and, second, that the males 

 in most species possess an excess of energy which manifests 

 itself in extra-growths, extra-development of pigment, 

 plumage, etc., and that displays by the males of special move- 

 ments, sound-making, etc., are direct effects or manifesta- 

 tions of sexual excitation. To these explanations should be 

 added the rather far-fetched one of Emery, who believes that 

 many cases of secondary sexual differences are explained by 

 the sudden appearance (mutation) of another form of male 

 or female, the persistence for a while of the two forms side 

 by side, as now exists in numerous dimorphic species (espe- 

 cially among insects), and then the gradual dying out (kill- 

 ing out by natural selection) of one of the two old original 

 forms (the one like the other sex), thus leaving the other, 

 or aberrant form. The ideas of Cunningham, 12 who does 

 not believe that any selection of fortuitous variation can 

 account for secondary sexual characters, may also be re- 

 ferred to. In a book of over three hundred pages this 

 author lists and describes according to principal animal 

 groups a host of secondary sexual characters, and pro- 

 poses a theory to account for them. 'The direct effects," 

 writes Cunningham, "of regularly recurrent stimulations are 



