But though the eye is perhaps more richly supplied with muscles, blood-vessels, and nerves than any other organ, yet, presumably, these structures here require the same kind of nutriment as they do elsewhere in the body; and since they are relatively minute they must, however functionally active, require a relatively minute quantity of nutriment—a quantity far too little when diverted to other organs to be important as a factor in survival. Therefore if the nutrition supplied to the eye is held to be such a drain on the resources of the organism that the saving of it appreciably affects the survival rate, we must suppose that the material supplied to its peculiar structures is physiologically extremely expensive. But as to this Mr. Spencer writes—
"If any one remembers how relatively enormous are the eyes of a fish just out of the egg—a pair of eyes with a body and head attached; if he then remembers that every egg contains material for such a pair of eyes, he will see that eye-material constitutes a very considerable part of the fish's roe; and that, since the female fish provides this quantity every year, it cannot be expensive" (p. 65).
In the proteus in particular eye-material cannot be expensive, for, since the animal does not exercise vision, the material is not used up, and therefore has not to be replaced. I think therefore that the
