60 THE USE AND NATURE 



portion of the anterior and upper surface of the animal, 

 in which aggregations of pigment occur, becomes more 

 than usually sensitive to light. A dark body passing in 

 front of such a region alters or gives rise to certain 

 molecular changes therein, and these molecular changes 

 (produced by large or small, near or remote, bodies) 

 differing among themselves, become capable of exciting 

 dissimilar impressions which the organism is gradually 

 attuned to discriminate. The existence of such a power 

 of discrimination in this, as in all other like cases, is 

 indicated by the creature's capability of responding to 

 impressions of this order by definite muscular movements 

 as when the Oyster, having the valves of its shell 

 apart, instantly closes them as soon as a shadow falls upon 

 certain pigment-specks, or so-called ' eyes,' at the edge of 

 its mantle.* 



This beginning of visual impressions truly enough shows 

 itself as a very exalted appreciation of tactile impressions ; 

 and, inasmuch as such an appreciation of the presence 

 of near bodies would in so many instances be quickly 

 followed by a more gross mechanical contact, the rudimen- 

 tary visual impression is, as H. Spencer happily puts it, a 

 kind of " anticipatory touch." From a simple beginning 

 of this kind, in which bodies only slightly separated from 

 the impressible foci excite certain general or only vaguely 

 specialized impressions corresponding to light and shade, 

 organs of Sight at once more elaborate and more impres- 

 sible gradually appear. To rudimentary aggregations of 

 pigment, in some animals transparent media are added, 



* Owen says (" Comp. Anat. of the Invert. Animals," p. 512) : 

 " Carlisle first showed that oysters were sensible of light ; having 

 observed that they closed their valves when the shadow of an 

 approaching boat was thrown forwards so as to cover them, before 

 any undulation of the water could have reached them." 



