198 SUMMARY. 



conditions of existence, so often insisted on by the illus- 

 trious Cuvier, is fully embraced by the principle of natural 

 selection. For natural selection acts by either now adapt- 

 ing the varying parts of each being to its organic and in- 

 organic conditions of life; or by having adapted them, 

 during past periods of time: the adaptations being aided in 

 many cases by the increased use or disuse of parts, being 

 affected by the direct action of the external conditions of 

 life, and subjected in all cases to the several laws of growth 

 and variation. Hence, in fact, the law of the Conditions 

 of Existence is the higher law; as it includes, through the 

 inheritance of former variations and adaptations, that of 

 Unity of Type. 



