XIII. 

 SECONDARY SEXUAL DIFFERENCES. 



It is admitted by Mr. Darwin and other evolution- 

 ists, that there are many peculiarities of structure and 

 instinct among animals which cannot be explained on 

 the theory of natural selection. 



Among these are secondary sexual differences — that 

 is, differences which exist aside from the sexual 

 organs themselves. 



These differences between the sexes may be in size, 

 the males being generally, but not always, the larger; 

 in color, the males in many cases being the more 

 highly colored, the reverse, however, being sometimes 

 true; in the presence of wings in one sex and their 

 absence in the other; in the possession of peculiarly 

 modified legs or of various peculiar appendages ; in 

 the presence or absence of voice, or of the power of 

 song, or of other means of making sound; in the 

 presence of certain appendages in the males alone, 

 such as spurs, horns, or other peculiar parts; in in- 

 stincts, and in many other respects. 



Mr. Darwin claims that many of the differences 

 between the sexes can be accounted for by sexual 

 selection, that is, by the choice of the female through 

 many generations. 



The theory of sexual selection involves the assump- 

 tion that the females are in some way called on to 

 make a choice between rival males, and that they have 

 an instinct which impels them to do so. Certain con- 

 ditions would be necessary in order that this choice 



might be exercised. 



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