OTHER THEORIES OF SPECIES-FORMING. 353 



perfected, adaptive structures? A male cricket has the 

 veins at the base of one wing-cover curiously and com- 

 plexly modified in course and in superficial structure, while 

 the veins of the other wing-cover are also modified in a way 

 differing from but exactly correlated with the venation of 

 the first wing, the whole specialisation resulting in a com- 

 bination of file, scraper, and vibrating membrane to form the 

 effective musical instrument of the insect. Can such an 

 adaptive structural modification be conceived to be a sudden 

 bursting forth or result of superabundant growth-force? 

 And many of the secondary sexual characters are of this 

 class of complex adaptive specialisations. The growth- 

 force explanation can, at best, explain but few of the various 

 categories of sexual dimorphism. Some explanation more 

 directive in its character is needed for these others. 



Even more restricted in its application, and less con- 

 vincing in the assumptions at its very base, is the curious 

 replacement theory of Emery. 30 This investi- 

 of th^origm of* 7 " ator believes that sexual selection can explain 

 secondary sexual but few if any cases of sexual dimorphism, and 



characters, 



would explain these other cases largely by the 

 sudden appearance (mutation or sport) of a second form 

 of male or female, the persistence for a while of the two 

 forms side by side, as now exemplified by numerous dimor- 

 phic or polymorphic (or di- or polychromatic) species, and 

 then the gradual or sudden dying out (killing out by selec- 

 tion?) of the older original form (the one resembling the 

 other sex), thus leaving the once dimorphic sex represented 

 only by the newer aberrant form. While such an explana- 

 tion may possibly explain a few cases of extreme sexual 

 dimorphism or dichromatism, it certainly will not do for 

 the many cases of secondary sexual difference constituted 

 by the existence in one sex of some one or few particular 

 adaptive specialisations for music-making, scent-producing, 

 or weapon-forming, not possessed by the other sex. 



