Chap. VI.] MODES OP TRANSITION. 229 



Modes of Transition. 



If it could be demonstrated that any complex organ 

 existed, which could not possibly have been formed by 

 numerous, successive, slight modifications, my theory 

 would absolutely break down. 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, accord- 

 ing to the theory, 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 developed; 

 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 Co- 

 bites the alimentary canal respires, digests, and ex- 

 cretes. In the Hydra, the animal may be turned in- 

 side out, and the exterior surface will then digest and 

 the stomach respire. In such cases natural selection 

 might specialise, if any advantage were thus gained, the 

 whole or part of an organ, which had previously per- 

 formed 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 

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