Origin of Species Variorum

These same organs at first appear to offer another and far more serious difficulty; for they occur in .. about a dozen kinds of fish, of which several are widely remote in their affinities. Generally when the same organ is found in several members of the same class, especially if in members having very different habits of life, we may attribute its presence to inheritance from a common ancestor; and its absence in some of the members to .. loss through disuse or natural selection. So that, if the electric organs had been inherited from some one ancient progenitor, .. we might have expected that all electric fishes would have been specially related to each other; but this is far from the case. Nor does geology at all lead to the belief that .. most fishes formerly possessed electric organs, which .. their modified descendants have now lost. But when we look closer to the subject, we find in the several fishes provided with electric organs that these are situated in different parts of the body,—that they differ in construction, as in the arrangement of the plates, and, according to Pacini, in the process or means by which the electricity is excited,—and lastly, in the requisite nervous power (and this is perhaps the most important of all the differences) being supplied through different nerves from widely different sources. Hence in the several remotely allied fishes furnished with electric organs, these cannot be considered as homologous, but only as analogous in function. Consequently there is no reason to suppose that they have been inherited from a common progenitor; for had this been the case they would have closely resembled each other in all respects. Thus the greater difficulty disappears, leaving only the lesser yet still great difficulty; namely, by what graduated steps these organs have arisen and been developed in each separate fish.