The electric organs offer another and even more serious difficulty;
for they occur in only about a dozen fishes, of which several are
widely remote in their affinities. Generally when the same organ
appears 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 its loss through disuse or natural selection. But if the
electric organs had been inherited from one ancient progenitor thus
provided, we might have expected that all electric fishes would have
been specially related to each other. Nor does geology at all lead to
the belief that formerly most fishes had electric organs, which most
of their modified descendants have lost. The presence of luminous
organs in a few insects, belonging to different families and orders,
offers a parallel case of difficulty. Other cases could be given; for
instance in plants, the very curious contrivance of a mass of
pollen-grains, borne on a foot-stalk with a sticky gland at the end,
is the same in Orchis and Asclepias,--genera almost as remote as
possible amongst flowering plants. In all these cases of two very
distinct species furnished with apparently the same anomalous organ,
it should be observed that, although the general appearance and
function of the organ may be the same, yet some fundamental difference
can generally be detected.
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