114 DARWINISM AND HUMAN LIFE 



favourable conditions of food-supply will no 

 longer suffice permanently to change the direction 

 of the variation." 



If, in such a case, the determinant be that of 

 a useful structure, then the ordinary process of 

 natural selection will remove the individual; but 

 if the weakened determinant be that of a useless 

 organ it wiU continue getting weaker generation 

 after generation. 



" In most cases the fluctuations will counteract 

 one another, because the passive streams of 

 nutriment soon change; but in many cases the 

 limit from which a return is possible will be passed, 

 and then the determinants concerned will continue 

 to vary in the same direction till they attain 

 positive or negative selection- value. At this stage 

 personal selection intervenes and sets aside the 

 variation if it is disadvantageous, or favours — 

 that is to say, preserves — it if it is advantageous. 

 But the determinant of a useless organ is un- 

 influenced by personal selection, and, as experience 

 shows, it sinks downwards ; that is, the organ 

 that corresponds to it degenerates very slowly 

 but uninterruptedly till, after what must obviously 

 be an immense stretch of time, it disappears from 

 the germ-plasm altogether." Thus " germinal 

 selection supplies the stones out of which personal 

 selection builds her temples and palaces : adapta- 

 tions." ^ 



The theory is, of course, entirely hjrpothetical, 

 dealing as it does with the invisible, but it enables 

 us to formulate a large number of facts. 



Vaeiational Stimuli. — Some of the most in- 

 teresting and striking work of the last dozen years 



1 " The Evolution Theory," by Weismaim (1904), vol ii. 



