CHAPTER XIX 



Orthogenesis 



Variations occurring in determined sequence. — Exaggerated de- 

 velopment of certain structures; giant reptiles; the tusks of 

 the mammoth and of the Babirussa, the antlers of the Irish 

 stag, the protective colouring of the Kallima. — Eimer's 

 orthogenesis and his laws of organic growth. — Cope's ar- 

 chaesthetism ; the role of consciousness. — Naegeli's views. 



ONE of the gravest objections formulated against 

 the theory of natural selection as the main fac- 

 tor in evolution, was based upon the nature of initial 

 variations. Those variations are supposed to be of 

 many kinds, purely accidental, and tending towards 

 all possible directions; they do not serve any purpose 

 until natural selection acts upon them and fosters the 

 most useful ones. 



On the other hand, a number of facts seem to indi- 

 cate that the development of certain organs and of 

 certain structures follows a well-defined line, regard- 

 less of whatever usefulness they may present. In 

 certain cases, we even observe that an organ, useful at 

 a certain stage of its evolution, may, by developing 

 continuously in the same direction, become positively 

 harmful and cause a species to die out instead of pro- 



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