170 ON THE ORIGIN OF SPECIES 



with a nerve sensitive to light beneath, and then sup- 

 pose every part of this layer to be continually changing 

 slowly in density, so as to separate into layers of differ- 

 ent densities and thicknesses, placed at different dis- 

 tances from each other, and with the surfaces of each 

 layer slowly changing in form. Further we must sup- 

 pose that there is a power always intently watching 

 each slight accidental alteration in the transparent 

 layers ; and carefully selecting each alteration which, 

 under varied circumstances, may in any way, or in any 

 degree, tend to produce a distincter image. We must 

 suppose each new state of the instrument to be 

 multiplied by the million ; and each to be preserved 

 till a better be produced, and then the old ones to be 

 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 on 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 ? 



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 my theory, there has been much extinction. Or 

 again, if we look to an organ common to all the mem- 

 bers of a large class, for in this latter case the organ 

 must have been first formed at an extremely 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 



