Fig. 119.—Electric organ of the Skate. The left-hand drawing (I) represents the entire organ (natural size) of a full-grown R. radiata. This is a small skate, which rarely exceeds 50 centms. in length; but in the large R. batis, the organ may exceed two feet in length. The other drawings represent single muscle-fibres in successive stages of transition. In the first of the series (II) the motor plate, and the nerves connected with it, have already been considerably enlarged. In the other three specimens, the fibre becomes more and more club-like, and eventually cup-like. These changes of shape are expressive of great changes of structure, as may be seen in the last of the series (V), where the shallow cup is seen in partial section. The electric plate lines the concavity of the cup, and is richly supplied with nerves (only a few of which are represented in the last drawing); the thick walls of the cup are composed of muscular fibres, the striation of which is distinctly visible.
Fig. 120.—Electric cells of Raia radiata. The drawing on the left represents one of the clubs magnified, as in the preceding wood-cut. The drawing on the right represents a number of these clubs, less highly magnified, in situ.
Again, it cannot be answered that the principle of correlation may be drawn upon in mitigation of the difficulty. The structure of the electric organ is far too elaborate, far too specialized, and far too obviously directed to a particular end, to admit of our conceivably supposing it due to any accidental correlation with structural changes going on elsewhere. Even as regards the initial changes of muscle-elements into electrical-elements, I do not think the principle of correlation can be reasonably adduced by way of explanation; for, as shown in the illustrations, even this initial change is most extraordinarily peculiar, elaborate, and specialized. But, be this as it may, I am perfectly certain that the principle of correlation cannot possibly be adduced to explain the subsequent association of these electrical elements into an electric battery, actuated by a special nervous mechanism of enormous size and elaboration—unless of course, the progress of such a structure were assumed to have been throughout of some utility. Under this supposition, however, the principle of correlation would be forsaken in favour of that of natural selection; and we should again be in the presence of the same difficulty as that with which we started.
But now, and further, if we do thus abandon correlation in favour of natural selection, and therefore if for the sake of saving an hypothesis we assume that the organ as it now stands must be of some use to the existing skate, we should still have to face the question—Of what conceivable use can those initial stages of its formation have been, when first the muscle-elements began to be changed into the very different electrical-elements, and when therefore they became useless as muscles while not yet capable of performing even so much of the electrical function as they now perform?
Lastly, we must remember that not only have we here the most highly specialized, the most complex, and altogether the most elaboratively adaptive organ in the animal kingdom; but also that in the formation of this structure there has been needed an altogether unparalleled expenditure of the most physiologically expensive of all materials—namely, nervous tissue. Whether estimated by volume or by weight, the quantity of nervous tissue which is consumed in the electric organ of the skate is in excess of all the rest of the nervous system put together. It is needless to say that nowhere else in the animal kingdom—except, of course, in other electric fishes—is there any approach to so enormous a development of nervous tissue for the discharge of a special function. Therefore, as nervous tissue is, physiologically speaking, the most valuable of all materials, we are forced to conclude that natural selection ought strongly to have opposed the evolution of such organs, unless from the first moment of their inception, and throughout the whole course of their development, they were of some such paramount importance as biologically to justify so unexampled an expenditure. Yet this paramount importance does not admit of being so much as surmised, even where the organ has already attained the size and degree of elaboration which it presents in the skate.
In view of all these considerations taken together, I freely confess that the difficulty presented by this case appears to me of a magnitude and importance altogether unequalled by that of any other single case—or any series of cases—which has hitherto been encountered by the theory of natural selection. So that, if there were many other cases of the like kind to be met with in nature, I should myself at once allow that the theory of natural selection would have to be discarded. But inasmuch as this particular case stands so far entirely by itself, and therefore out of analogy with thousands, or even millions, of other cases throughout the whole range of organic nature, I am constrained to feel it more probable that the electric organ of the skate will some day admit of being marshalled under the general law of natural selection—in just the same way as proved to be the case with the conspicuous colouring of those caterpillars, which, as explained in the last chapter, at one time seemed to constitute a serious difficulty to the theory, and yet, through a better knowledge of all the relations involved, has now come to constitute one of the strongest witnesses in its favour.
I have now stated all the objections of any importance which have hitherto been brought against the theory of natural selection, excepting three, which I left to be dealt with together because they form a logically connected group. With a brief consideration of these, therefore, I will bring this chapter to a close.