Chap. VI.] SUMMARY. 259 



having performed at the same time the same function, 

 the one having been perfected whilst aided by the 

 other, must often have largely facilitated 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 examined, 

 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 accumulated by means of 

 natural selection. In many other cases, modifications 

 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 retained (as the tail of an aquatic animal by its 

 terrestrial descendants), 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 



