DIFFICULTIES OF THE THEORY 169 



perfected while aided by the other, must often have largely facili- 

 tated transitions. 



We have seen that in two beings widely remote from each other 

 in the natural scale, organs serving for the same purpose and in 

 external appearance closely similar may have been separately and 

 independently formed; but when such organs are closely exam- 

 ined, essential differences in their structure can almost always be 

 detected; and this naturally follows from the principle of natural 

 selection. On the other hand, the common rule throughout nature 

 is infinite diversity of structure for gaining the same end ; and this 

 again naturally follows from the same great principle. 



In many cases we are far too ignorant to be enabled to assert 

 that a part or organ is so unimportant for the welfare of a species, 

 that modifications in its structure could not have been slowly ac- 

 cumulated by means of natural selection. In many other cases, 

 modifications are probably the direct result of the laws of varia- 

 tion or of growth, independently of any good having been thus 

 gained. But even such structures have often, as we may feel as- 

 sured, 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 retained (as the tail of an aquatic animal by its 

 terrestrial descendants), though it has become of such small im- 

 portance 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 coun- 

 try. For in 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 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 



