CHAP. VI.] SUMMARY. 155 



concluding that none can have existed, for the metamorphoses of 

 many organs show what wonderful changes in function are at 

 least possible. For instance, a swimbladder has apparently been 

 converted into an air-breathing lung. The same organ having 

 performed simultaneously very different functions, and then 

 having been in part or in whole specialised for one function ; and 

 two 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 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 through- 

 out 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 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 country. 

 For in the larger country there will have existed more individuals 

 and more diversified forms, and the competition will have been 

 severer, and thus the standard of perfection will have been 



