CHAPTER VI. KINETOGENESIS. 



T TNDER the head of kinetogenesis (development by 

 LJ motion) comes the consideration of the effect of 

 use and disuse. Use necessarily conditions the evolu- 

 tion of useful characters. These characters are such 

 by reason of their adaptation to the life-functions of 

 the beings which possess them. It is perfectly well 

 known, however, that all plants and animals possess 

 more or less numerous peculiarities which are not use- 

 ful to their possessors. Such are the mammae of male 

 animals; the incisions forming palmate or pectinate 

 leaves and petals of plants ; rudimental organs of all 

 kinds ; great elongation of the vertebral column, espe- 

 cially of the caudal series in certain species ; patches 

 of color, or of hairs, at particular places, etc. These, 

 and many others may % be arranged in divisions accord- 

 ing to their probable origin, as follows : 



I. Excess of growth energy. 



Examples: the recurved tusks of the mammoth, babirussa, 

 etc. ; the elongate feathers of some birds, etc. 



II. Defect of growth-energy. 



i. Atavism: examples; the tritubercular superior molar of 

 certain races of man. 



