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 

 came 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 de- 

 tected; 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 fol- 

 lows 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, modifica- 

 tions 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 assured, been subsequently taken ad- 

 vantage of, and still further modified, for the good of 

 species under new conditions of life. We may, also, be- 

 lieve that a part formerly of high importance has fre- 

 quently 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 



