CHAPTER VI 



ORIGIN OF NON-SEXUAL CHARACTERS : THE 

 PHENOMENA OF MUTATION 



ACCORDING to the theory here advocated, modifica- 

 tions produced by external stimuli in the soma will 

 also be inherited in some slight degree in each genera- 

 tion when they have no relation to sex or reproduc- 

 tion. In this case the habits and the stimuli which 

 they involve will be common to both sexes, and the 

 hormones given off by the hypertrophied tissues will 

 act upon the corresponding determinants in the 

 gametocytes. The modifications thus produced will 

 therefore be related to habits, and the theory 

 will include all adaptations of structure to function, 

 but other characters may also be included which are 

 the result of stimuli and yet have no function or 

 utility. 



The majority of evolutionists in recent years have 

 taught that influences exerted through the soma 

 have no effect on the determinants in the chromo- 

 somes of the gametes, that all hereditary variations 

 are gametogenic and none somatogenic. Mendelians 

 believe that evolution has been due to the appearance 

 of characters or factors of the same kind as those 

 which distinguish varieties in cultivated organisms, 

 and which are the subject of then: experiments, but 

 they have found a difficulty, as already mentioned 

 in Chapter II, in forming any idea of the origin of a 



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