Therefore, as far as this objection to the theory of natural selection is concerned—or the allegation that homologous structures occur in different divisions of organic nature—not only does it fall to the ground, but positively becomes itself converted into one of the strongest arguments in favour of the theory. As soon as the allegation is found to be baseless, the very fact that it cannot be brought to bear upon any one of all the millions of adaptive structures in organic nature becomes a fact of vast significance on the opposite side.
The next difficulty to which I shall allude is that of explaining by the theory of natural selection the preservation of the first beginnings of structures which are then useless, though afterwards, when more fully developed, they become useful. For it belongs to the very essence of the theory of natural selection, that a structure must be supposed already useful before it can come under the influence of natural selection: therefore the theory seems incapable of explaining the origin and conservation of incipient organs, or organs which are not yet sufficiently developed to be of any service to the organisms presenting them.
This objection is one that has been advanced by all the critics of Darwinism; but has been presented with most ability and force by the Duke of Argyll. I will therefore state it in his words.
If the doctrine of evolution be true—that is to say, if all organic creatures have been developed by ordinary generation from parents—then it follows of necessity that the primæval germs must have contained potentially the whole succeeding series. Moreover, if that series has been developed gradually and very slowly, it follows, also as a matter of necessity, that every modification of structure must have been functionless at first, when it began to appear.... Things cannot be selected until they have first been produced. Nor can any structure be selected by utility in the struggle for existence until it has not only been produced, but has been so far perfected as to actually be used.
The Duke proceeds to argue that all adaptive structures must therefore originally have been due to special design: in the earlier stages of their development they must all have been what he calls “prophetic germs.” Not yet themselves of any use, and therefore not yet capable of being improved by natural selection, both in their origin and in the first stages (at all events) of their development, they must be regarded as intentionally preparatory to the various uses which they subsequently acquire.
Now this argument, forcible as it appears at first sight, is really at fault both in its premiss and in its conclusion. By which I mean that, in the first place the premiss is not true, and, in the next place, that even if it were, the conclusion would not necessarily follow. The premiss is, “that every modification of structure must have been functionless at first, when it began to appear;” and the conclusion is, that, quâ functionless, such a modification cannot have been caused by natural selection. I will consider these two points separately.
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.
So much, then, for the first consideration which serves to invalidate the Duke’s premiss. The second consideration is, that very often an organ which began by being useful for the performance of one function, after having been fully developed for the performance of that function, finds itself, so to speak, accidentally fitted to the performance of some other and even more important function, which it thereupon begins to discharge, and so to undergo a new course of adaptive development. In such cases, and so far as the new function is concerned, the difficulty touching the first inception of an organ does not apply; for here the organ has already been built up by natural selection for one purpose, before it begins to discharge the other. As an example of such a case we may take the lung of an air-breathing animal. Originally the lung was a swim-bladder, or float, and as such it was of use to the aquatic ancestors of terrestrial animals. But as these ancestors gradually became more and more amphibious in their habits, the swim-bladder began more and more to discharge the function of a lung, and so to take a wholly new point of departure as regards its developmental history. But clearly there is here no difficulty with regard to the inception of its new function, because the organ was already well developed for one purpose before it began to serve another. Or, to take only one additional example, there are few structures in the animal kingdom so remarkable in respect of adaptation as is the wing of a bird or a bat; and at first sight it might well appear that a wing could be of no conceivable use until it had already acquired enormous proportional dimensions, as well as an immense amount of special elaboration as to its general form, size of muscle, amount of blood-supply, and so on. For, obviously, not until it had attained all these things could it even begin to raise the animal in the air. But observe how fallacious is this argument. Although it is perfectly true that a wing could be of no use as a wing until sufficiently developed to serve the purpose of flight, this is merely to say that until it has become a wing it is no use as a wing. It does not, however, follow that on this account it was of no prior use for any other purpose. The first modifications of the fore-limb which ended in its becoming an organ of flight may very well have been due to adapting it as an organ for increased rapidity of locomotion of other kinds—whether on land as in the case of its now degenerated form in the ostrich, or in water as in the case of the expanded fins of fish. Indeed, we may see the actual process of transition from the one function to the other in the case of “flying-fish.” Here the progressive expansion of the pectoral fins must certainly have been always of use for continuously promoting rapidity of locomotion through water; and thus natural selection may have continuously increased their development until they now begin to serve also as wings for carrying the animal a short distance through air. Again, in the case of the so-called flying squirrels we find the limbs united to the body by means of large extensions of the skin, so-that when jumping from one tree to another the animal is able to sustain itself through a long distance in the air by merely spreading out its limbs, and thus allowing the skin-extensions to act after the manner of a parachute. Here, of course, we have not yet got a wing, any more than we have in the case of the flying-fish; but we have the foundations laid for the possible development of a future wing, upon a somewhat similar plan as that which has been so wonderfully perfected in the case of bats. And through all the stages of progressive expansion which the skin of the squirrel has undergone, the expansion has been of use, even though it has not yet so much as begun to acquire the distinctive functions of a wing. Here, then, there is obviously nothing “prophetic” in the matter, any more than there was in the case of the swim-bladder and the lung, or in that of the nerve-ending and the eye. In short, it is the business of natural selection to secure the highest available degree of adaptation for the time being; and, in doing this, it not unfrequently happens that an extreme development of a structure in one direction (produced by natural selection for the sake of better and better adapting the structure to perform some particular function) ends by beginning to adapt it to the performance of some other function. And, whenever this happens to be the case, natural selection forthwith begins to act upon the structure, so to speak, from a new point of departure.