ASPECTS OF KINETIC EVOLUTION 373 



The complete separation of species into two sexes is the con- 

 dition obtaining in all the higher animals, both vertebrates and 

 arthropods, as well as in many of the lower animals, and in 

 numerous plants. It has been found recently that even among 

 the moulds and other lower fungi the plant body, or mycelium, 

 is of two kinds, and that spores are produced only when these 

 are brought together. 



Secondary sexual characters are of two kinds, or may be so 

 considered : (i) Those which are accessory to reproductive 

 processes, or assist in caring for the seeds, eggs, or young, 

 such as the mammas of the higher animals ; (2) those which 

 are merely the result of accumulation of differences which add 

 to the heterism or internal diversity of the species, such as the 

 manes, beards, tail-feathers or sexual differences of color or 

 form which are of no use in reproduction or in the environ- 

 mental relations of the species. 



The environmental uselessness of many sexual differences is 

 an obvious and well known fact. Not only do the two sexes 

 generally occupy exactly the same environment with equal suc- 

 cess, but the presence or absence of many sexual character- 

 istics may have no practical significance for the individual. 

 Some varieties of mankind are beardless ; some have beards 

 only late in life, and some have beards in early manhood, but 

 cut them off without appreciable detriment. The uselessness 

 of such characters is shown even more strikingly in certain 

 species of beetles. Some of the males are scarcely distin- 

 guishable externally from the females, while others have the 

 head or thorax fantastically modified by the growth of long, 

 heavy, antler-like processes. It is easy to understand that for 

 all the males to be thus encumbered might be a serious handi- 

 cap to the species. 



It may be that selection will help to explain why such fea- 

 tures commonly pertain to the male sex. Great diversity among 

 the females would interfere with recognition by males unless 

 their instincts were modified in a corresponding manner. More- 

 over, variation is the more practicable in the male sex because 

 the extent of the coordination necessary among the bodily or- 

 gans is not so great. Variation, which in the females might 



