186 SUMMARY. 



known or unknown, it must on this theory be strictly 

 true. 



It is generally acknowledged that all organic beings have 

 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 existence, 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 vari- 

 ations and adaptations, that of Unity of Type. 



