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.