produced by "waves" of light emanating from a distance, and is thus brought into mediate contact with certain distant objects. A refinement of the organs of taste may also occur whereby bodies possessing sapid qualities are capable of impressing organisms still at a distance. Just as vision, in fact, is, in its most elementary phases, a sort of "anticipatory touch," so is smell a kind of anticipatory taste. Yet the two cases are not altogether similar. In vision, the contact—if it may be so termed—with the distant body is mediate, through the intervention of ethereal undulations; while in smell we have to do with a case of immediate contact, not with the distant body itself of course, but with extremely minute particles which it gives off on all sides. An "emission" theory serves to explain the diffusion of odors, though it will not hold for the diffusion of light. From what I have said it may be inferred that, as regards the delicacy of their respective physical causes, the sense of smell occupies an intermediate position between taste and sight.
It is regarded as a matter of certainty by naturalists that such creatures as spiders, crustacea, insects, and the higher mollusks, are capable of being impressed in some way by odors, and that their actions are to a certain extent regulated by such impressions. We have, however, no definite knowledge concerning the parts of the surface which in these, and perhaps in still lower organisms, are attuned to receive such influences. Although a rudimentary sense of smell seems unquestionably to be possessed by such aquatic forms of the invertebrata as Crustacea and the higher mollusks, it is, perhaps, a sense-endowment which generally exists in a more developed and more varied form among air-breathing animals. In whatever forms of life it may be met with, however, the sense of smell seems to be very largely indeed related to the detection and capture of food; so, that, in these relations, it comes to the aid of the already-existing senses of sight, touch, and taste, though it has the peculiarity of being scarcely otherwise called into activity among the invertebrata.
Although we have no positive knowledge concerning the situation of the organs of smell among invertebrate animals, there is good reason for believing that in crustacea they are to be found at the base of the antennules; that in cephalopods they are represented by two little fossæ in the neighborhood of the eyes; and that in insects a power of appreciating odors is possibly possessed either by the antennae themselves, or by a pair of fossæ near their bases. Another cephalic organ has also been referred to as possibly endowed with a power of being impressed by odors. Thus Owen says: "The application, by the common house-fly, of the sheath of its proboscis to particles of solid or liquid food, before it imbibes them, is an action closely analogous to the scenting of food by the nose in higher animals; and, as it is by the odorous qualities, much more than by the form of the surface, that we judge of the fitness of substances for food, it is more reasonable to
