Cretecisms of Theory of Natural Selection. 353 
And being thus useful from the very moment of its 
inception, it would afterwards be gradually improved 
as variations of more and more utility presented them- 
selves, until not only would finer and finer degrees of 
difference between light and shade become perceptible, 
but even the outlines of solid bodies would begin to 
be appreciated. And so on, stage by stage, till from 
an ordinary nerve-ending in the skin is evolved the 
eye of an eagle. 
Moreover, in this particular instance there is very 
good reason to suppose that the modification of the 
cutaneous nerves in question began by a progressive 
increase in their sensitiveness to temperature. Wher- 
ever dark pigment happened to be deposited in the 
skin—and we know that in all animals it is apt to be 
deposited in points and patches, as it were by accident, 
or without any “prophecy” as to future uses,—the 
cutaneous nerves in its vicinity would be better able 
to appreciate the difference between sun and shade in 
respect of temperature, even though as yet there were 
no change at all in these cutaneous nerves tending to 
make them responsive to light. Now it is easy to see 
how, from such a purely accidental beginning, natural 
selection would have had from the first sufficient 
material to act upon. It being of advantage to a 
lowly creature that it should distinguish with more 
and more delicacy, or with more and more rapidity, 
between light and darkness by means of its thermal 
sensations, the pigment spots in the skin would be 
rendered permanent by natural selection, while the 
nerves in that region would by the same agency 
be rendered more and more specialized as organs 
adapted to perceive changes of temperature, until 
* Aa 
