164 MODES OF TRANSITION. 



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 alteration, 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 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, *nore especially if we look to much- 

 isolated species, around which, according 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 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 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 selection 

 might specialize, 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 con- 

 structed flowers ; and if such plants were to produce one 

 kind alone, a great change would be effected with compare 



