98 GLAUCUS ; Oil, 



given hiin 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 necessary, namely, the repetition of 

 the same form, slightly modified, in totally differ- 

 ent 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 anat- 

 omy. 



Look, again, at those sea-slugs. One, some 

 three inches long, of a bright lemon-yellow, 

 clouded with purple ; another of a dingy gray ; t 



* Flssurdla f/rceca. 



t Doris iubcrculata and hilineata. 



