218 ORIGIN OF SPECIES 



been formed on two great laws — Unity of Type, and the 

 Conditions of Existence. By unity of type is meant that 

 fundamental agreement in structure which we see in organic 

 beings of the same class, and which is quite independent of 

 their habits of life. On my theory, unity of type is explained 

 by unity of descent. The expression of conditions of exist- 

 ence, so often insisted on by the illustrious Cuvier, is fully 

 embraced 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 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. 



