24 HEREDITY [ch. 



be indirectly affected, giving rise to variation in the 

 offspring; in such a case, however, there is no 

 necessity that the effect on the offspring should be in 

 any way similar to the direct effect of the conditions 

 on the parent. Nothing is know^n of the nature of 

 possible effects of environment on the germ-cells ; 

 the action may possibly be effective immediately and 

 give rise to variability in the next generation, or it 

 may be that the effects are cumulative and only cause 

 visible changes after several generations have been 

 exposed to the same influences. Galton [13] suggested 

 that the organism may have a certain * stability,' but 

 that influences acting for several generations may 

 have a cumulative effect which will gradually alter the 

 equilibrium until it is finally upset and falls into a 

 new condition of stability, giving rise to an apparently 

 sudden variation. A chemical analogy may make this 

 clearer. If litmus is added to an alkaline solution its 

 colour will be blue. Acid may now be added drop 

 by drop to neutralize the alkali, and suddenly, when 

 the solution becomes acid, the litmus turns red. 

 Examples of variation of which this may possibly be 

 an analogy will be given below. 



With regard to the action of environment on the 

 body many facts are known, but it is not certain that 

 they really have any bearing on the question of the 

 origin of variation. For variations so produced are 

 'acquired characters,' and in many cases at least 



