WONDERS OF THE SHORE 1369 



to certain anatomical peculiarities, he needed one 

 aperture more than a limpet; so one, if you will ex- 

 amine, has been given him at the top of his shell 

 (Fissurella graeca). 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 im- 

 pressions of sense; and of a custom in nature which 

 makes this caution so necessary, namely, the repeti- 

 tion 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 marvelous 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 further 

 removed, dies out altogether; placed there it would 

 seem at first sight merely to keep up the family like- 

 ness. I am half jesting; that can not 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 gray (Doris tuberculata 

 and bilineata) ; another exquisite little creature of a 

 pearly French white, furred all over the back with 

 what seem arms, but are really gills, of ringed white 

 and gray and black (Eolis papilosa). Put that yel- 

 low 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- 



