Chap. VI.] SUMMARY. 261 



by the principle of natural selection. For natural 

 selection acts by either now adapting the varying 

 parts of each being to its organic and inorganic con- 

 ditions 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. 



