Plants, like animals, as the result of tissue change, have certain used-up or waste matters to get out of the way. Animals have special excretory organs for the purpose; waste matter remains in the flesh and blood of dead animals. In plants are found a large number of powerful volatile oils, alkaloids, bitter resins, etc. Many of these are, in all probability, excretory products of no assimilative value to the plant. Certain volatile oils may attract insects, and in obtaining nectar from flowers insects assist fertilisation. Agreeable volatile oils and flavouring substances in fruits attract birds and animals. The eating of the fruits cause the seeds, which are uninjured by passing through the digestive system, to be disseminated over wide areas to the advantage of the plant species. On the other hand, nauseous and poisonous alkaloids, oils, resins, etc., serve as a protection against the attacks of browsing animals, birds, caterpillars, snails, etc. These nauseous substances are most abundant in the bark, husk, skin and outer parts. It is commonly supposed that the food on which each animal, including man, subsists, is especially produced by Nature for the purpose. This is an error, for each species of plant and animal lives for itself alone, and protects itself, with more or less success, against destruction by its competitors and enemies. Each species of animal selects from its surroundings such food as is most suitable. Such food may not be theoretically perfect; that is, it may not contain the maximum of nourishment free from innutritious matter; but during the long period of evolution, each species of animal has become possessed of organs suited to its environment. If to such animals be given food containing less indigestible matter, or food which is more readily digested by laboratory tests made independently of the living animal, their digestive system will be thrown out of gear, become clogged up or refuse to work properly, just as the furnace of a steam boiler, made to burn coal, will act badly with wood or petroleum. Many scientific men have overlooked this fact, and have endeavoured to produce food substances for general consumption, in the most concentrated and soluble form, thinking such food would be more easily assimilated. The Volatile and Essential Oils are contained in minute quantity in a very large number of animal and vegetable foods. They contribute in part to the flavour of fruits. They are the cause of the pungency and aroma of mustard, horse-radish, cloves, nutmegs, cinnamon, caraway seeds, mint, sage and other spices. Onions contain a notable quantity. When extracted the essential oils become powerful drugs. In moderate quantities they are stomachic and carminative, in larger quantities irritant and emetic. Condiments and spices not only add flavour to food, but stimulate the secretion of gastric juice and peristaltic movement.