260 SUMMARY. [Chap. VL 



useful or even indispensable, or again highly injurious 

 to another species, but in all cases at the same time 

 useful to the possessor. In each well-stocked country 

 natural selection acts through the competition of the 

 inhabitants, and consequently leads to success in the 

 battle for life, only in accordance with the standard of 

 that particular country. Hence the inhabitants of one 

 country, generally the smaller one, often yield to the 

 inhabitants of another and generally the larger country. 

 For in the larger country there will have existed more 

 individuals and more diversified forms, and the com- 

 petition will have been severer, and thus the standard 

 of perfection will have been rendered higher. Natural 

 selection will not necessarily lead to absolute perfection; 

 nor, as far as we can judge by our limited faculties, can 

 absolute perfection be everywhere predicated. 



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 to the present inhabitants alone of the world, is not 

 strictly correct; but if we include all those of past times, 

 whether 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 in- 

 sisted on by the illustrious Cuvier, is fully embraced by 

 the principle of natural selection. For natural selection 



