132 Evolution and Adaptation 
surfaces of each 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 watch- 
ing each slight alteration in the transparent layers ; and care- 
fully preserving each which, under varied circumstances, in 
any way or in any degree, tends to produce a distincter 
image. We must suppose each new state of the instru- 
ment 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 multiply them almost infi- 
nitely, 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.” 
We may conclude in Darwin’s own words : — 
“To arrive, however, at a just conclusion regarding the 
formation of the eye, with all its marvellous yet not abso- 
lutely perfect characters, it is indispensable that the reason 
should conquer the imagination ; but I have felt the difficulty 
far too keenly to be surprised at others hesitating to extend 
the principle of natural selection to so startling a length.” 
The electric organs, present in several fish, offer a case 
of special difficulty to the selection theory. When well 
developed, as in the Torpedo and in Gymnotus, it is conceiv- 
able that it may serve as an organ of defence, but in other 
forms the shock is so weak that it is not to be supposed that 
it can have any such function. Romanes, who in many ways 
was one of the stanchest followers of Darwin, admits that, so 
far as he can see, the evolution of the electric organs cannot 
be explained by the selection theory. Darwin offers no 
explanation, but bases his defence on the grounds that we do 
not know of what use this organ can be to the animal. 
