Chap. VI. OKGANS OF EXTREME PERFECTION. 189 



part of this layer to be continually changing slowly in 

 density, so as to separate into layers of different densi- 

 ties and thicknesses, placed at different distances from 

 each other, and with the surfaces of each layer slowly 

 changing in form. Further we must suppose that there 

 is a power always intently watching each slight acci- 

 dental alteration in the transparent layers ; and carefully 

 selecting each alteration which, under varied circum- 

 stances, may in any way, or in any degree, tend to pro- 

 duce 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 selec- 

 tion 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 



