184 ORIGIN OF SPECIES 



layer slowly changing in form. Further we must suppose 

 that there is a power, represented 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 sup- 

 pose each new state of the instrument to be multiplied 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 mul- 

 tiply 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 ex- 

 isted, which could not possibly have been formed by numer- 

 ous, successive, slight modifications, my theory would abso- 

 lutely break down. But I can find out no such case. No 

 doubt many organs exist of which we do not know the tran- 

 sitional grades, more especially if we look to much-isolated 

 species, round 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 orean 

 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 Cobites the alimentary canal respires, digests, 

 and excretes. In the Hydra, the animal may be turned in- 



