THE WONDERS OF THE SHORE. 129 
all: he is of quite a different family and structure ; 
but, on the whole, a limpet-like shell would suit 
him well enough, so he had one given him: never- 
theless, 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.1 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 some- 
times (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 
1 Fissurella greca, Plate X. fig. 5. 
