pedo, instead of being peculiar, may be only another form of
the discharge which attends upon the action of muscle and motor nerve."
Beyond this we cannot at present go in the way of explanation; but as
we know so little about the uses of these organs, and as we know nothing
about the habits and structure of the progenitors of the existing
electric fishes, it would be extremely bold to maintain that no
serviceable transitions are possible by which these organs might have
been gradually developed.

These organs appear at first 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. 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 generally 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 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
differen