DIFFICULTIES OF THE THE0R7 285 



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 examined, essential differ- 

 ences 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 through- 

 out nature is infinite diversity of structure for gaining 

 the same end; and this again naturally follows from the 

 same great principle. 



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

 'to assert that a part or organ is so unimportant for the 

 iwelfare of a species that modifications in its structure 

 jiould not have been slowly accumulated by means of 

 lutural selection. In many other cases, modifications are 

 )robably the direct result of the laws of variation or 

 )f growth, independently of any good having been thus 

 jained. But even such structures have often, as we may 

 eel assured, been subsequently taken advantage of, and 

 till further modified, for the good of species under new 

 onditions of life. We may, also, believe that a part 

 Drmerly of high importance has frequently been retained 

 IS the tail of an aquatic animal by its terrestrial descend- 

 ej |nts), though it has become of such small importance 

 at it could not, in its present state, have been acquired 

 7 means of natural selection. 



Natural selection can produce nothing in one species 

 >r the exclusive good or injury of another; though it 

 ay well produce parts, organs, and excretions highly 

 ii) feeful or even indispensable, or again highly injurious to 

 lother species, but in all cases at the same time useful 



