viii BOTANICAL EVIDENCE 385 



Moreover, by " form " I mean here essentially the form which 

 grows from the germ by natural necessity as the result of 

 regular growth. But with the words " during its individual 

 life " reference is made also to the alterations of form which 

 organisms undergo during life and which may be inherited. 

 These inheritable modifications produce at first little or 

 scarcely any visible external alteration ; they are internal, 

 dynamic. Nevertheless, as I have stated, external features 

 due to age, and even mutilations, as well as diseases which 

 affect form, may be inherited. 



By the expression " spontaneous internal modifications " I 

 mean the alterations produced by constitutional causes, among 

 them some senile changes, which help to determine form. 



Vegetable physiology supplies the most palpable evidence 

 that the forms of organisms are determined by the influence 

 of external forces on protoplasm, by acquired and inherited 

 properties. 



Is it not the influence of light which determines the 

 direction of growth in plants ? Do not light and air produce 

 the direction, expansion, and position of leaves on which 

 nutrition depends ? Is it not the force of gravity which 

 determines the form of roots and causes them to grow towards 

 the centre of the earth ? Has not nutrition the most profound 

 influence on the form of all parts of plants ? Is not the 

 influence of temperature equally important ? 



To which the reply will be made : that if all external 

 stimuli were removed all these effects would disappear, that 

 they themselves are not inherited, but only the predispositions 

 towards them. I have already remarked previously that every 

 " predisposition " presupposes a molecular modification. With 

 the predispositions are inherited the modifications which 

 alone render possible definite directions of growth under the 

 influence of external stimuli. It is certainly true that if all 

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