ADAPTATION. 243 



be an invalid inference. Without assuming fixity of species, 

 we find good reasons for anticipating that kind and degree of 

 stability which is observed. We find grounds for concluding, 

 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 generations. 



