Chap. VI. SUMMAKY. 205 



simultaneously very different functions, and then having 

 been specialised for one function ; and two very distinct 

 organs 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 are far too ignorant, in almost every case, to be 

 enabled to assert that any part or organ is so unim- 

 portant for the welfare of a species, that modifications 

 in its structure could not have been slowly accumulated 

 by means of natural selection. But we may confidently 

 believe that many modifications, wholly due to the laws 

 of growth, and at first in no way advantageous to a spe- 

 cies, have been subsequently taken advantage of by the 

 still further modified descendants of this species. We 

 may, also, believe that a part formerly of high import- 

 ance has often 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 natural selec- 

 tion, — a power which acts solely by the preservation of 

 profitable variations in the struggle for life. 



Natural selection will 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 highly injurious to 

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

 to the owner. Natural selection in each well-stocked 

 country, must act chiefly through the competition of 

 the inhabitants one with another, and consequently will 

 produce perfection, or strength in the battle for life, only 

 according to the standard of that country. Hence the 

 inhabitants of one country, generally the smaller one, 

 will often yield, as we see they do yield, to the inha- 

 bitants of another and generally larger country. For in 



