DIFFICULTIES OF THE THEORY 255 



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 glass as the works of the 

 Creator are to those of man? 



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 gra- 

 dations of some kind. Numerous cases could be given 

 among 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 ali- 

 mentary canal respires, digests, and excretes. In the 

 Hydra, the animal may be turned inside out, and the 



