MUTATION 167 



tera. The same green species sometimes have a 

 brown form, too. In these eases the new char- 

 acters of pinkness and of brownness have un- 

 doubtedly arisen suddenly and no intergrades are 

 known. Of the common May beetle, I am in- 

 formed by Professor Forbes, no less than forty- 

 two forms are known from Illinois alone, several 

 of them difficult to distinguish by superficial 

 characters, all of them readily separated by ref- 

 erence to the copulatory structures, which are dif- 

 ferent in the various species and in the two sexes. 

 " These structures are so constant," writes Pro- 

 fessor Forbes, "that one of my assistants who 

 has handled over ten thousand specimens of one 

 species for determination, says that they are all 

 like castings from the same mold." There are 

 features of this case that certainly look like mu- 

 tation; particularly the large number of species 

 in a small area separated by non-intergrading 

 differences in one variable organ. But, as Pro- 

 fessor Forbes suggests, there is one difficulty in 

 the way of seeing how the differences could have 

 arisen by mutation : the copulating organs in each 

 species are mechanically adapted to each other; 

 and this requires that a coincident and coadaptive 

 mutation occur in the two sexes. But this is a 

 true difficulty only so long as we conceive the 

 entire organ to be a single unit character. There 

 is, however, as little reason for so conceiving it 

 as for regarding the human hand as one unit 

 character instead of many units. The evolu- 



