Not only is the discrimination of individuals by the sense of smell a very astonishing thing, but so also is the obvious fact that the total amount of odoriferous matter which is sufficient to give a definite and discriminative sensation through the organ of smell is of a minuteness beyond all calculation or conception. These two facts—the almost infinite individual diversity of smell and the almost infinite minuteness of the particles exciting it—render it very difficult to form a satisfactory conclusion as to the nature of those particles. It has been from time to time suggested that the end organs of the olfactory nerves may be excited, not by chemically active particles, but by "rays," olfactive undulations comparable to those of light. Physicists have not yet been able to deal with the problem, but the recent discoveries and theories as to radio-active bodies such as radium may possibly lead to some more plausible theory as to the diffusion and minuteness of odorous particles than any which has yet been formulated. An example of the minuteness of odoriferous particles is afforded by a piece of musk which for ten years in succession has given off into the changing air of an ordinary room "particles" causing a readily recognised smell of musk, and yet is found at the end of that time to have lost no weight, that is to say, no weight which can be appreciated by the finest chemical balance. An analogy (I say only an analogy, a resemblance) to this is furnished by a pinch of the salt known as radium chloride, no bigger than a rape-seed, and enclosed in a glass tube, which will continue for months and years to emit penetrating particles producing continuously without cessation most obvious luminous and electrical effects upon distant objects, the particles being so minute that no loss of weight can be detected in the pinch of salt from which they are given off.
The sense of smell is of service to animals—
(1) In avoiding enemies and noxious things.
(2) In tracing and following and discriminating prey or other food.
(3) In recognising members of their own species and individuals of their own herd or troop, and in finding their own young and their own nests.
(4) In seeking individuals of the opposite sex at the breeding season.
It is in connection with the last of these services that we come across some of the most curious observations as to the production and perception of odorous particles. Butterflies and moths and some other insects have olfactory organs in the ends of the antennæ and the "palps" about the mouth. The perfumes of flowers have been developed so as to attract insects by the sense of smell, as their colours have been also developed to attract insects by the eye. The insects serve the flowers by carrying the fertilizing pollen from one flower to another, and thus promoting cross-fertilization among separate individual plants of the same species. But probably concurrently with this has grown up the production of perfume by the scales on the wings of moths and butterflies—perfumes which have the most powerful attraction for the opposite sex of the same species. Curiously enough (for these perfumes might very well exist without being detected by man) some of the perfumes produced by butterflies are "smellable" by man. That of the green-veined white is described as resembling the agreeable odour of the lemon verbena. It is produced by certain scales on the front border of the hinder wings of the male insects, and not at all by the females, who are, however, attracted by it, and flutter around the sweet-smelling male. Other male butterflies produce a scent like that of sweet briar, others like honeysuckle, others like jasmine, and so attract the females. Other butterflies are known which produce repulsive odours, and so protect themselves from being eaten by birds and lizards. Again, there are moths (for instance, the emperor moth, Saturnia), the females of which produce a perfume which attracts the males, and is of far-reaching power. The French entomologist, Fabre, placed one of these female moths in a box covered with net-gauze, and left it in a room with open window, facing the countryside. In less than an hour the room was full of male emperor moths—more than a hundred arrived, although none had been previously visible in the neighbourhood. They crowded over the box, and even afterwards, when the female moth had been removed, the perfume remained in the box, and the male moths eagerly sought it. The perfume must have carried far from the room where the female was, out into the woods where it was perceived, and followed up to its source by the male moths.
Such perfumes are very generally produced by little pockets or glands in the skin, the secretion having, in the case of insects, birds and mammals, an oily nature. In mammals they are largely produced by both males and females, and serve to attract the sexes to one another. Hairs are situated close to the minute odoriferous glands and serve an important part in accumulating and diffusing the characteristic perfume. Musk and civet are of this nature, and it is a significant fact that these substances are used as perfumes by human beings. It would seem as though mankind had lost either the power of satisfactorily perceiving the perfumes naturally produced by the human skin, or that the production of such perfumes had for some reason diminished. Either condition would account for the use by mankind of the perfumes of other animals and of flowers. There are a variety of odorous substances produced by different parts of the human body, of which some are agreeable and others disagreeable. One of the most curious facts in regard to odorous bodies is the close resemblance between agreeable and repulsive odours, and the readiness with which the judgment of human beings may pronounce the same odour agreeable at one period or place, and disagreeable at another. There also seems to be a "dulling" of the power to perceive an odour which is a consequence of constant exposure to that odour. Thus the Chinese say that Europeans all smell unpleasantly, the odour resembling that of sheep, although we do not observe it; whilst Europeans notice and dislike the smell of the negro, a smell of the existence of which he is unaware. The blood of animals, including that of man, has, when freshly shed, a smell peculiar to the species, which has not, however, any resemblance to that of the skin or of the waxy glands of the same animal.
It seems that in regard to the exercise of the sense of smell by man, we must distinguish not only greater from less acuteness and variety of perception, but in the case of this sense-organ, as in regard to the others, we must distinguish "unconscious" from "conscious" sensation. All our movements are guided and determined by sensations to touch and sight, and to some extent, of hearing, of which we are unconscious. A vast amount of our sense-experience comes to us and is recorded without our having consciousness of anything of the kind going on. It is probable that the world of smells in which a dog with a fine olfactive sense lives, produces little or nothing in the dog's mind which is equivalent to our conscious perception of degrees of agreeable and disagreeable odours. The dog is simply attracted and repulsed in this direction and in that by the operation of his olfactive organs, without, so to speak, giving any attention to the sensation which is guiding him or being "aware" of it. No doubt at times, and with special intensities of smell, he is, in his way, conscious of a specific sensation. It is probable that whilst man's general acuteness in perceiving and discriminating smells has dwindled (as has that of the apes) in comparison with what it was in his remote animal ancestry, yet he retains a large inherited capacity of unconscious smell-sense, which most of us are unable to recognise, although it is there, operating in ourselves unknown to us and unobserved. The consciousness of smell-sensations is what we value and talk of. It does not extend to the more primal smell-excitations, except in extraordinary individuals. Thus, it seems to be not improbable that we are attracted or repelled by other human individuals by the unconscious operation upon us of attractive or repulsive odours, and that the unaccountable liking or disliking which we sometimes experience in regard to other individuals is due to perfumes and odours emanating from such persons, which act upon us through our olfactory organs without our being conscious of the fact. It seems that we can thus arrive at a probable explanation of the universality of the habit of kissing, and of "what is that thing we call a kiss." It is not consciously used among civilised populations as a deliberate attempt to smell the person kissed, but it nevertheless serves to allow the unconscious exercise of smell-preference, testing, and selection, with which are mingled, more or less frequently, moments of conscious appreciation of the complex of odours appertaining as an individual quality to the person kissed.