e 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
sub