Page:Popular Science Monthly Volume 9.djvu/238

218 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, medusæ and various kinds of worms, appear to hold converse with the outside world. Touches and tastes are the names which we apply to the subjective 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 with 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 he 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 follow 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 irritated 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 region gives rise to certain molecular changes therein, and these molecular 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