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.
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