On the other hand, the candid inquirer must admit, that when in some creatures well-developed special organs exist, while in their immediate kinsfolk (if I may use the term) no trace of kindred structures has been discovered, there herein exists a grave objection to any theory of unity of type resulting from community of descent. This difficulty appears to have been more obvious to Mr. Darwin than to most of the reviewers who have undertaken to criticize his views, or at least has been more clearly and fairly stated by the former than by the latter; and he speaks of the case of the electrical organs of fishes as one of special difficulty.
The presence of modified, atrophied, or rudimentary organs, constitutes one of the strongest arguments in favour of Mr. Darwin's theory; for the supposition is as unsatisfactory as it is improbable, that such organs are the result of what would seem a whimsical exercise of creative power in framing an organ merely for the sake of symmetry. The total absence, however, of any trace of even an altered or rudimentary organ representing a structure known to exist in certain members of a group, would afford good testimony against the theory of descent; as it would be at least in the highest degree improbable that such a structure should not have its homological representative existing in some form in the immediate members of the same family.
Considering, therefore, that on the theory of Mr. Darwin it was in the highest degree improbable that the electric organs of the Torpedo were totally absent in the Skates, I undertook a careful search, with the view of following out their homologies; determining to do so by tracing the nerves corresponding with those which go to supply the batteries of the Torpedo. I have thus been led to make out the bodies which I conceive to be the true homologues of the Torpedo's wondrous organs; and the anatomical position and peculiarities of which I shall briefly point out. If the skin be removed from the fore part of the back of a common Skate, the following parts will be readily found, a short distance behind the temporal orifice:—1st. That band of the so-called muciferous tubes which runs inwards and a little backwards from a point external and anterior to the gills; 2nd. The dorsal aspect of the branchial chambers; and, 3rd. The little snout-muscle, which ends in a long delicate tendon, running forwards. Let the little fleshy belly of the snout-muscle be raised and drawn outwards, and the band of tubes dissected up and drawn forwards, in the angle between them will be found the body sought for; it will not, however, be very apparent to the naked eye; but if brushed over with some tolerably strong acetic acid, it will become quite distinctly visible. It will be found to be more than an inch long in an ordinary sized fish, wedged in between the occipital muscles internally and the gills externally, covered superficially by the snout-muscle and tubes already mentioned, and dipping down so as to reach the branches of the vagus going to the branchial arches. Its upper surface is triangular, the apex behind the base in front, in contact with one of the large jaw muscles. When made evident by the aid of acetic acid, this little body is seen to consist of a
