Chap. VI. OKGANS OF EXTREME PERFECTION. 
189 
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 multi¬ 
plied by the million; and each to be preserved till a 
better be produced, and then the old ones to be de¬ 
stroyed. In living bodies, variation will cause the 
slight alterations, generation will multiply them almost 
infinitely, and natural selection will pick out with un¬ 
erring 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 
