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 at the subject more closely, 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 being supplied with nerves proceeding from different sources, and this is perhaps the most important of all the differences.

Hence in the several 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 difficulty of an organ, apparently the same, arising in several remotely allied species, disappears, leaving only the lesser yet still great difficulty: namely, by what graduated steps these organs have been developed in each separate group of fishes.
The luminous organs which occur in a few insects, belonging to widely different families, and which are situated in different parts of the body, offer, under our present state of ignorance, a difficulty almost exactly parallel with that of the electric organs.