had one given him: nevertheless, owing to certain anatomical
peculiarities, he needed one aperture more than a limpet; so one,
if you will examine, has been given him at the top of his shell.
(15) This is one instance among a thousand of the way in which a
scientific knowledge of objects must not obey, but run counter to,
the impressions of sense; and of a custom in nature which makes
this caution so necessary, namely, the repetition of the same form,
slightly modified, in totally different animals, sometimes as if to
avoid waste, (for why should not the same conception be used in two
different cases, if it will suit in both?) and sometimes (more
marvellous by far) when an organ, fully developed and useful in one
species, appears in a cognate species but feeble, useless, and, as
it were, abortive; and gradually, in species still farther removed,
dies out altogether; placed there, it would seem, at first sight,
merely to keep up the family likeness. I am half jesting; that
cannot be the only reason, perhaps not the reason at all; but the
fact is one of the most curious, and notorious also, in comparative
anatomy.

Look, again, at those sea-slugs. One, some three inches long, of a
bright lemon-yellow, clouded with purple; another of a dingy grey;
(16) another exquisite little creature of a pearly French White,
(17) furred all over the back with what seem arms, but are really
gills, of ringed white and grey and black. Put that yellow one
into water, and from his head, above the eyes, arise two serrated
horns, while from the after-part of his back springs a circular
Prince-of-Wales's-feather of gills, - they are almost exactly like
those which we saw just now in the white Cucumaria. Yes; here is
another instance of the same custom of repetition. The Cucumaria
is a low radiate animal - the sea-slug a far higher mollusc; and