260 STDDIARY. [Chap. VI. 



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 competition will have been severer, and thus 

 the standard of perfection will have been rendered 

 higher. Xatural 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. " Xatura 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 tins 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 



