200 THE INDUCTIONS OF BIOLOGY. 



a priori, that an adaptive change of structure, will soon reach 

 a point beyond which further adaptation will be slow ; for 

 concluding that when the modifying cause has been but 

 a short time in action, the modification generated, will be 

 evanescent ; for concluding that a modifying cause acting 

 even for many generations, will do but little towards per- 

 manently altering the organic equilibrium of a race ; 

 and for concluding that on the cessations of such cause, its 

 effects will become unapparent in the course of a few gener- 

 ations. 



