THE WONDERS OF THE SHORE. 117 



by his sucking-foot ? A limpet ? jN'ot at 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 : nevertheless, owing to 

 certain anatomical peculiarities, he needed one aper- 

 ture more than a limpet ; so one, if you will examine, 

 has been given him at the top of his shell* 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 neces- 

 sary, 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 concep- 

 tion 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, use- 

 less, 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 



* Fissiirella grseca, Plate X. Fig. 5. 



