tebrata.

Now, in the particular case of the skate, the organ is situated in the
tail, where it is of a spindle-like form, measuring, in a large fish,
about two feet in length by about an inch in diameter at the middle of
the spindle. Although its structure is throughout as complex and perfect
as that of the electric organ in _Gymnotus_ or _Torpedo_, its smaller
size does not admit of its generating a sufficient amount of
electricity to yield a discharge that can be felt by the hand.
Nevertheless, that it does discharge under suitable stimulation has been
proved by Professor Burdon Sanderson by means of a telephone; for he
found that every time he stimulated the animal its electrical discharge
was rendered audible by the telephone. Here, then, the difficulty
arises. For of what conceivable use is such an organ to its possessor?
We can scarcely suppose that any aquatic animal is more sensitive to
electric shocks than is the human hand; and even if such were the case,
a discharge of so feeble a kind taking place in water would be
short-circuited in the immediate vicinity of the skate itself. So there
can be no doubt that such weak discharges as the skate is able to
deliver must be wholly imperceptible alike to prey and to enemies. Yet
for the delivery of such discharges there is provided an organ of such
high peculiarity and huge complexity, that, regarded as a piece of
living mechanism, it deserves to rank as at once the most extremely
specialized and the most highly elaborated structure in the whole animal
kingdom. Thousands of separately formed elements are ranged in row after
row, all electrically insulated one from another, and packed away into
the smallest possible space, with the obvious end, or purpose, of
conspiring together for the simultaneous delivery of an electric shock.
Nevertheless, the shock when delivered is, as we have just seen, too
slight to be of any conceivable use to the skate. Therefore it appears
impossible to suggest how this astonishing structure--much more
astonishing, in my opinion, than the human eye or the human hand--can
ever have been begun, or afterwards developed, by means of natural
selection. For if it be not even yet of any conceivable use to its
possessor, clearly thus far survival of the fittest can have had nothing
to do with its formation. On the other hand, seeing that electric organs
when of larger size, as in the _Gymnotus_ and _Torpedo_, are of obvious
use to their possessors