166 SUMMARY. [Chap. VI, 



cations are probably the direct result of the laws of variation or of 

 growth, independently of any good having been thus gained. But 

 even such structures have often, as we may feel assured, been 

 subsequently taken advantage of, and still further modified, for the 

 good of species under new conditions of life. We may, also, believe 

 that a part formerly of high importance has frequently been re- 

 tained (as the tail of an aquatic animal by its terrestrial descend- 

 ants), though it has become of such small importance that it could 

 not, in its present state, have been acquired by means of natural 

 selection. 



Natural selection can produce nothing in one species for the 

 exclusive good or injury of another ; though it may well produce 

 parts, organs, and excretions highly 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. 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 Ave 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 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 



