. VI.] MODES OF TRANSITION. 137 



each other, and with the surfaces of each layer slowly changing 

 in form. Further we must suppose that there is a power, repre- 

 sented by natural selection or the survival of the fittest, always 

 intently watching each slight alteration in the transparent layers ; 

 and carefully preserving each which, under varied circumstances, 

 in any way or in any degree, tends to produce a distincter image. 

 We must suppose each new state of the instrument to be multi- 

 plied by the million ; each to be preserved until a better one is 

 produced, and then the old ones to be all destroyed. In living 

 bodies, variation will cause the slight alterations, generation will 

 multiply them almost infinitely, and natural selection will pick 

 out with unerring skill each improvement. Let this process go 

 on for millions of years ; and during each year on millions of 

 individuals of many kinds ; and may we not believe that a living . 

 optical instrument might thus be formed as superior to one of >C 

 glass, as the works of the Creator are to those of man 'i 



Modes of Transition. 



If it could be demonstrated that any complex organ existed, 

 which could not possibly have been formed by numerous, succes-(r^i 

 sive, slight modifications, my theory would absolutely break down, v 

 But I can find out no such case. No doubt many organs exist of 

 which we do not know the transitional grades, more especially if 

 we look to much-isolated species, round which, according to the 

 theoiy, there has been much extinction. Or again, if we take an 

 organ common to all the members of a class, for in this latter case 

 the organ must have been originally formed at a remote period, 

 since which all the many members of the class have been de- 

 veloped ; and in order to discover the early transitional grades 

 through which the organ has passed, we should have to look to 

 very ancient ancestral forms, long since become extinct. 



We should be extremely cautious in concluding that an organ 

 could not have been formed by transitional gradations of some kind. 

 Numerous cases could be given amongst the lower animals of the 

 same organ performing at the same time wholly distinct functions ; 

 thus in the larva of the dragon-fly and in the fish Cobites the 

 alimentary canal respires, digests, and excretes. In the Hydra, 

 the animal may be turned inside out, and the exterior surface will 

 then digest and the stomach respire. In such cases natural selec- 

 tion might specialise, if any advantage were thus gained, the whole 

 or part of an organ, which had previously performed two functions, 

 for one function alone, and thus by insensible steps greatly change 

 its nature. Many plants are known which regularly produce at 

 the same time differently constructed flowers ; and ft such plants 

 were to produce one kind alone, a great change would be effected 

 with comparative suddenness in the character of the species, li 



