206 DIFFICULTIES ON THEORY. Chap. VI. 



the larger country there will have existed more indi- 

 viduals, and more diversified forms, and the competition 

 will have been severer, and thus the standard of perfec- 

 tion will have been rendered higher. Natural selection 

 will not necessarily produce absolute perfection ; nor, as 

 far as we can judge by our limited faculties, can absolute 

 perfection be everywhere found. 



On the theory of natural selection we can clearly 

 understand the full meaning of that old canon in natural 

 history, " Natura non facit saltum." This canon, if 

 we look only to the present inhabitants of the world, is 

 not strictly correct, but if we include all those of past 

 times, it must by my 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 prin- 

 ciple 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 long-past periods of time : the 

 adaptations being aided in some cases by use and dis- 

 use, being slightly affected by the direct action of the 

 external conditions of life, and being in all cases sub- 

 jected to the several laws of growth. Hence, in fact, 

 the law of the Conditions of Existence is the higher 

 law ; as it includes, through the inheritance of former 

 adaptations, that of Unity of Type. 



