First as to the premiss, it is not true that every modification of structure must necessarily be functionless when it first begins to appear. There are two very good reasons why such should not be the case in all instances, even if it should be the case in some. For, as a matter of observable fact, a very large proportional number of incipient organs are useful from the very moment of their inception. Take, for example, what is perhaps the most wonderful instance of refined mechanism in nature--the eye of a vertebrated animal. Comparative anatomy and embryology combine to testify that this organ had its origin in modifications of the endings of the ordinary nerves of the skin. Now it is evident that from the very first any modification of a cutaneous nerve whereby it was rendered able, in however small a degree, to be differently affected by light and by darkness would be of benefit to the creature presenting it; for the creature would thus be able to seek the one and shun the other according to the requirements of its life. 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 themselves, 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. Wherever 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 from the stage of responding to the thermal rays of the non-luminous spectrum alone, they become capable of responding also to luminous.