218 THE POPULAR SCIENCE MONTHLY. 



a mode of impressibility does, doubtless, exist in many other of the 

 lower forms of life. Impressions of the two orders already referred to 

 more or less distinct from one another are those by which alone 

 multitudes of the lowest forms of animal life, such as polyps, medusae 

 and various kinds of worms, appear to hold converse with the outside 

 world. Touches and tastes are the names which ice apply to the sub- 

 jective effects of such impressions; and, though it is impossible at 

 present wholly to ignore this point of view, or to use language which 

 is not colored by it, I do not now wish to say anything with regard 

 to the nature or intensity of the feelings that may be associated w r ith 

 corresponding impressions in the lower animals. The reader must for 

 the present look rather to the objective effects of these impressions, and 

 in so doing lie will learn that these become organically associated with 

 a nervous mechanism by whose intermediation they are able to evoke 

 distinctive movements of a responsive nature. 



Seeing, however, that tactile and gustatory impressions can only 

 be made by actual contact of external bodies with the specialized 

 parts of an organism, such impressions are not of a kind to excite 

 movements in quest of food ; although they may lead to correlated 

 movements of parts adjacent to those which are touched, as when all 

 the tentacles of a sea-anemone close round a body that has come into 

 contact with some one of them. This effect is due to a radiation of 

 the primitive stimulus, and we may see in such a set of actions only 

 a more rapid and slightly more complex result than is known to fol- 

 low the irritation of one of the peripheral tentacles on the leaf of a 

 sun-dew. In the latter case the bending of the tentacle actually irri- 

 tated is slowly followed by the bending of others under the influence 

 of an internally diffused stimulus. 



Movements in actual quest of food may, however, be excited in 

 other animal organisms by impressions which suffice to bring them 

 into relation with more or less distant bodies. The way is paved for 

 this result when some 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 re- 

 gion gives rise to certain molecular changes therein, and these molec- 

 ular changes differing among themselves become capable of exciting 

 distinctive impressions in the organism which it gradually becomes 

 attuned to discriminate. The power of discrimination in this, as in 

 all other cases, is indicated by the organism's capability of responding 

 to impressions by definite muscular movements as when the oyster, 

 with the valves of its shell apart, instantly closes them if a shadow is 

 projected over certain sensitive pigment-specks or so-called " eyes " 

 at the edge of its mantle. 



This beginning of visual impressions truly enough shows itself as a 

 merely exalted appreciation of tactile impressions ; and, inasmuch as 

 such an appreciation of the presence of near bodies would in so many 



