14 FACTORS JN VARIATION. 



Had \ve the necessary knowledge to enable us fully 

 to analyse the complex of stimuli afforded by environment 

 — using the word in its widest sense — we should probably 

 be able to account for the origin of all variations which 

 cannot 1)e directly traced to Mendelian mechanisms. The 

 factors of what may be called radial evolution (outlined 

 by the writer in a former paper under the term radiogenesis) 

 would then be made clear. It is difficult to realise the 

 full significance and magnitude of the work done by such 

 experimenters as Jacques Loeb in analysing the mechanism 

 of animal conduct and in demonstrating the i-ole of 

 tropisms.* Loel) does not accept the inheritance of 

 acquired characters as adequately proved, but he boldly 

 states that '" the quantitative laws prevailing in the effect 

 of environment upon organisms leave no more room for 

 the interference of a " directing force " of the vitalist than 

 do the laws of the motion of the solar system "' (loe. cit., 

 H)l(i, ]). 317). 



Although Weismann in his great work. "" The Evolution 

 Theory." gives no support to neo-Lamarckian views, it is 

 notewortliy that in the second volume there is a whole 

 cha]itei' on "' Influence of Environment." He also recoi-ds 

 the influence on the germ-plasm of " very minute nutritive 

 changes "" (p. 196). 



The importance of food as a factor in variation is 

 generally recognised. Charles Darwin stated : — " Of all 

 the causes which induce variability, excess of food, whether 

 or not changed in nature, is probably the most powerful. "f 

 The great diversity to be foimd in land shells is well known, 

 and this is j^robably due to divergencies in diet. The 

 contours of the shells of Helicidae correspond with the 

 shai)e of the visceral sac. According to the experiments 

 of 8inu-oth, quoted by Weismann, { "' a change of diet ma_y 

 evoke many kinds of changes in the structure of the food- 

 canal, which may indirectly compel changes in the shell." 



*J. Locb : The Dynamics of Living Matter. 1906: The Organi.sra 

 as a Whole, 1910. Forced Movenient.s, Tropisnis and Animal Conduct, 

 1918. 



f Darwin : Variation of AnimaLs and Plants under Dome.stication, 

 II., 2nd edit., 1890, p. 244. 



JWeiHinann : The Evolution Theory, II., 1904, p. 302 



