SMELLS AND PERFUMES
The old saying, "_De gustibus non disputandum_," is based upon the
fact that both liking and the repulsion evinced by human beings for
different odours (including those odours which we call flavours) are
not matters of general agreement. Thus the smells of garlic and of
onions, and even of assafoetida, are to many men among the most
attractive and appetising in existence--to very many they are, on the
other hand, repulsive. High game, a certain kind of putrid fish
("Bombay ducks"), and again rotten cheese are attractive to many men
and offensive to as many more. Many animals revel in the smell and
flavour of carrion, and even of manure, which they devour. There are
well-known flowers which attract insects, not by the possession of the
sweet perfumes appreciated and extracted by mankind, but by a smell
like that of putrid meat, which so far misleads blue-bottle flies as
to cause them to lay their eggs on the reeking blossom. So diverse are
the tastes of men and animals in these matters that it is remarkable
when we find agreement among them, as, for instance, in the attraction
for butterflies of those delicate scents which also are agreeable to
ourselves in such flowers as the rose, the jasmine, the heliotrope and
the honeysuckle.
There seems to be no rule or principle at work by which smells can be
definitely classed as either pleasant or unpleasant. Even perfumes
carried by some of the inhabitants of Western Europe with the
intention of making themselves attractive to their fellow-citizens are
often repulsive to a certain proportion of those who come near them,
as, for instance, is the case with the extract of the East Indian herb
"patchouli." In regard to our other senses there is a general
agreement amongst mankind, which extends also to all animals, as to
what is agreeable and what is disagreeable. There are definite
mathematical laws as to harmony and melody in sound and colour which
affect animals and ourselves to a large extent similarly. Sweets are
agreeable and bitters are disagreeable, though it is the fact that the
snail, which loves sugar, recoils from saccharine, and there are
"mites" (_Acari_) which feed with avidity on bitter strychnine! Excess
of heat and of cold is disliked by animals and all men, whilst the
sense of touch is pleasurably or painfully affected in much the same
way in most men and animals, more than is the case with regard to any
other of the senses. The sense of smell depends upon immediate and
personal experience of "association" for the determination of pleasure
or pain, attraction or repulsion, as the result of its being called
into operation. It is a very general experience that odours are more
efficient in arousing memory than are mere colour effects or sounds.
Not only in animals with acutely developed olfactory powers, but also
in man, an odour--a peculiar perfume--will start a whole chain of
reminiscence when sight and sound have failed to do so. It is due to
this close association with memory (conscious or unconscious) that an
odour is agreeable or disagreeable.
In itself an odour is neither attractive nor repulsive. The acrid
fumes of sulphur, chlorine, ammonia, and such bodies are not simply
"odours" but corrosive chemical vapours, which act painfully upon the
nerves of common sensation within the air-passages of the nose and
throat and not exclusively, if at all, on the terminations of the
olfactory nerves. An odour--that which acts on the special nerves of
smell distributed in chambers of the nose--acquires its attractive or
its repulsive quality only as the result of mental association with
what is beneficial (suitable food, mates, friends, safety, home, the
nest), or with what is injurious (unsuitable food, poison, enemies,
danger, strange surroundings, solitude). Hence it is intelligible that
the man accustomed to garlic or onions in his food is strongly
attracted by their smell. So too the man whose tribe or companions
have learnt by necessity to eat slightly putrid meat, fish, and cheese
is attracted by their odour, though for others these odours are
associated rather with what is poisonous and injurious. The dislike of
the smell of sewer-gas and foul accumulations of refuse was not known
to former generations of men (even in European cities a couple of
hundred years ago) any more than it is to-day to the more unfortunate
poorer classes, to many modern savages, to hyenas, and several other
animals and birds which inhabit lairs and caves which they make foul.
The odour of putrescence has become actually painful and almost
intolerable to the more cleanly classes of mankind, owing to the
association with it, as the result of education, of fear of disease
and poisoning. Either conscious or unconscious association of an odour
with what is held, either as the result of tradition or through
personal experience, to be beneficial and of pleasant memory, or, on
the contrary, injurious and of painful connection, determines man's
liking for and choice or rejection of, odours and flavours. One can
account with fair success on this basis for one's own preferences and
dislikes in the matter.
On the other hand, odours exist in vast variety amongst plants and
animals which have not acquired any special association or
significance. We find that some organisms produce as a result of their
chemical life material which oxidises and gives out light and so these
organisms are "phosphorescent" without any consequence, good or bad,
to themselves. And then we come upon others (as, for instance, the
glow-worms and fire-flies) which have made use of this "accidental"
quality, and produce phosphorescent light in special organs so as to
attract the opposite sex. Again, we find that the red-coloured
oxygen-seizing crystalline substance haemoglobin exists in the blood of
a vast number of animals, and might as well be green or colourless for
all the good its colour does them. Yet here and there the splendid red
colour which this chemical gives to the blood becomes of great
importance as a "decoration," or "sex-ornament." The comb of the
domestic fowl, the wattles of the turkey, but above all the supreme
beauty of the human race--the cherry-red lips and the crimson-blushing
cheek of healthy youth--owe their wonderful colour to the red blood
which flows through them. So at last the redness, of the
oxygen-carrier is turned to account. So it must be also with odorous
substances. Many have been called into existence, but few have been
chosen in the long course of animal evolution and selected as the
important means of repulsion or attraction.
There are odorous substances attached to many of the lower animals
which seem to have no significance, but just happen to be the result
of necessary chemical changes, not aimed (so to speak) at their
production. Of course, it is very difficult to form a certain and
definite conclusion as to their uselessness as odours. For instance,
nearly all the sponges when fresh and filled with living protoplasm
have a curious smell which reminds one of that given off by a stick of
phosphorus. Marine sponges have it and so has the beautiful green or
flesh-coloured liver sponge (common on the wood of rafts and weirs in
the Thames). A rather uncommon marine worm, called _Balanoglossus_ or
the acorn worm, has a very strong and unpleasant smell like that of
iodoform. In neither case is the nature of the odorous body known, nor
its use to the animal suggested. Smelts smell like cucumbers: the
green-bone fish and the mackerel smell alike. One of the common
earth-worms has a strong aromatic smell, and the common snail, as well
as the sea-hare and one of the cuttle-fishes (_Eledone_), smells like
musk. Musk itself is produced, as a scent attracting the opposite sex,
by several animals--musk-deer, musk-sheep, musk-rats. I am not now
attempting to enumerate the well-recognised odours of animals such as
are extracted from them by man in order to "opsonize" himself, but am
pointing to the more obscure cases. There is not a very great or
marked variety in the odours of fishes; but reptiles with their dry,
oily skins give off various aromatic smells, none of which are valued
by man. Toads have distinct odours, and one kind (_Pelobates fuscus_,
or the heel-clawed toad), common in Europe, but not British, is known
locally as the garlic toad on account of its smell. There are amongst
carnivorous mammals various smells allied to that of civet which are
not so agreeable to man as that substance; for instance, the odour of
the fox and of the badger, and yet more celebrated, the terrible,
awe-inspiring smell of the fluid emitted in self-defence by the skunk
from a sac in the hinder part of the body. Horses, cows, goats, sheep,
and the giraffe have their distinctive odours. Many of the herbivorous
animals secrete a colourless fluid from large glands opening on or
near the feet, and also from a gland in front of the eye (similar
glands occur in other strange positions), which has not a smell
familiar to man--that is to say, not one which has been recognised and
described--yet seems to be readily "smelt" by the animals of its own
kind. The bats--especially the large frugivorous bats--have a very
unpleasant, frowsy smell.
An important fact about animal smells is that many which we might be
inclined to attribute to the animal which diffuses them, are really
due to the fermentative or putrefactive action of bacteria which swarm
on the skin and in the intestines of animals. It is often difficult to
decide how far a peculiar animal odour is due directly to a substance
secreted by the animal, and how far the odour of that substance is
modified or even entirely produced by the chemical changes set up in
secretions of the body-surface by bacteria. Several distinct repulsive
smells liable to occur on the human body are due to want of
cleanliness in destroying bacteria by proper antiseptics. The fatty
and waxy secretions of the skin are often decomposed by bacteria, even
before complete extrusion from the glands in which they are formed,
whilst the decomposition of food in the mouth and intestines by
bacteria alters materially both the natural odour of the animal's
breath and the smell of the intestinal contents. In young and healthy
animals in natural conditions there is some check--it is not easy to
say what--upon the putrefactive activities of the omnipresent
bacteria. The skin of a healthy young animal has a pleasant odour,
and its breath (notably in the case of the cow and the giraffe) is
naturally sweet-smelling. The same should be the case, under perfectly
healthy conditions, with human beings.
There is one important cause of animal odours and flavours upon which
I have not hitherto touched. Many animals acquire an odour or flavour
directly from the food upon which they feed. Certain odorous bodies
are in the food and are taken up into the blood of the consuming
animal unchanged, and are then thrown out by secreting glands on the
skin. This is the case with the odorous substance of onions. People do
not smell of onions after they have eaten them in consequence of
particles of onion remaining in the mouth. The volatile odoriferous
matter of the onion is absorbed into the blood. It passes out first
through the lungs and later through the small fat-forming glands in
the skin. It is difficult to ascertain how far animals derive their
odours in this way in a complete state from their food, and how far
they chemically construct them afresh by their own activity. No doubt
both processes occur; but in plants the odorous bodies are built up
entirely by the chemical action of the plant itself upon simple salts
of carbonic acid, ammonia and nitrates. Animals can certainly take
highly elaborated chemical bodies into their digestive organs without
destroying them and absorb them unchanged into the blood and deposit
them in the tissues. Thus the canary is made to take up the red colour
of cayenne pepper and deposit it in the feathers. Thus the green
oysters of Marennes acquire their colour from minute blue plants
(diatoms) on which they feed. And thus, too, the canvas-backed ducks
of the United States take into their tissues the odorous matter of
celery, and our own grouse the flavour of heather, whilst fish-eating
birds and whales in this way acquire a fishy taste. So, too, the
flounders and the eels of the Thames, and even salmon in muddy rivers,
acquire a taste like the smell of river mud. It is probable that many
of the odours of animals (but by no means all) are thus derived
directly from their food, or are produced by very slight changes of
the odorous bodies absorbed in food. Mutton and beef owe their savour
in some degree to the scents of the grasses on which sheep and oxen
feed. And it is not improbable that the sheep-like smell which the
Chinese detect in the European, comes to the latter direct from his
general use of the sheep as food.
Plants are the great chemical manufacturers in the world of life, and
second to them come our human industrial and scientific chemists. And
though we must claim for animals some power of manufacturing distinct
odorous bodies from inodorous nutritive matter assimilated by them, it
is probable that in many cases the odour which is characteristic of an
animal is derived by no very complicated change from odorous bodies
existing in its habitual food.
A curious case of a substance valued as perfume by civilised man, and
yet coming from a source whence sweet odours would hardly be expected,
is that which is known as "ambergris," or "ambre gris" (grey amber).
It is still used in the manufacture of esteemed perfumes, and is sold
at five guineas the ounce. It is found floating on the surface of the
ocean, and is a concretion of imperfectly digested matter from the
intestine of a whale--probably the sperm-whale. It is a grey, powdery
substance, and in it are embedded innumerable fragments of the horny
beaks and sucker-rings of cuttle-fishes--creatures which form the
chief food of the sperm-whale and other toothed whales. I have already
mentioned above that one of our common cuttle-fishes (the _Eledone
moschata_) has a strong odour of musk, and it is possible that
ambergris owes its perfume to the musk-like scent of the cuttle-fish
eaten by the whale in whose intestine it is formed. Another "smell"
which is extremely mysterious is that produced by two quartz-pebbles,
or even two rock-crystals, or two pebbles of flint or of corundum,
when rubbed one against the other. A flash of light is seen, and this
is accompanied by a very distinct smell, like that given out by
burning cotton-wool. It is demonstrated--by careful chemical cleaning
before the experiment--that this is not due to the presence of any
organic matter on or in the stones or crystals used. It seems to be an
exception to the rule that "odours" (as distinct from pungent vapours
or gases) are only produced by substances formed by plants or animals.
Perhaps that is not so completely a rule as I was inclined to think.
It is true that one can distinguish the "smells" of chlorine, of
bromine, and of iodine from one another. And there are statements
current as to the distinctive smells of metals--though they may
possibly be due to the action of the metals on organic matter. In any
case it seems, according to our present knowledge, that the smell
given out by the rubbing of pieces of silica (quartz, flint, etc.) is
due to particles of silica (oxide of silicon) volatilised by the heat
of friction, which are capable of acting specifically on the olfactory
sense-organ.