194 MOLLUSCA THEIR EYES. 



of the horn is drawn inwards, in a manner which resembles 

 the turning in of a stocking. The annular fibres which en- 

 circle the horn throughout the whole of its length, unfold 

 the internal part by successive contractions, and thus bring 

 back the eye to its external position. * 



The location of these organs has been varied according to 

 the wants and habits of the animal, and to the medium in 

 which it was destined to live ; but neither this medium nor 

 these habits have had the slightest share in producing the 

 variations ; a remark so obvious, that it may well seem to 

 you unnecessary, did not the language of too many natural- 

 ists imply something to the contrary, f The notion is akin 

 in absurdity to another doctrine which Lamarck in particular 

 has strenuously and long endeavoured to establish, viz. that 

 the eyes (and of course the same reasoning applies to the 

 organs of the other senses) are the products or manufacture 

 of strong desires on the creature's part to enjoy the blessing 

 of sight. " The production," he says, " of a new organ in 

 an animal body is the result of a new supervenient need 

 which continues to be felt, and of the new motion which this 

 need has originated and keeps up : " and as an illustration of 

 this law, which he is careful to print in italic letters, he 

 instances the snail, which, in crawling along, finds the neces- 

 sity of touching the bodies which are before it, makes efforts 

 to do so with some of the anterior prominences of the head, 

 and sends there every moment waves of nervous fluid and 

 of the other liquids. Now, the result of these repeated 

 tides on the points in question, is a gradual extension of 

 their nerves, a gradual growth of their substance, and the 

 ultimate budding and evolution of two or four tentacula. J 

 Do not think that I overstrain the doctrine, for indeed I trans- 



* Cuvier, Comp. Anat. trans, ii. 444 " We observe/' says Miiller, " in 

 the cavity of the terminal portion of the tentaculum, a little liquid which 

 seems to be of service in facilitating the evolution of the organ." 



f See a very objectionable passage quoted from Ferussac, in Zool. Jour- 

 nal, ii. 505. 



i Anim. sans Vert^bres, i. 185 188. The doctrine of Lucretius is anti- 

 podal to Lamarck's, and just as true : 



" Note here, Lucretius dares to teach 

 (As all our youth may learn from Creech) 

 That eyes were made, but could not view, 

 Nor hands embrace, nor feet pursue : 

 But heedless Nature did produce 

 The members first, and then the use. 

 What each must act was yet unknown, 

 Till all is moved by chance alone." PRIOR. 



