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.

The Alkaloids most used are those of tea, coffee, kola-nut, cocoa, coca, tobacco and opium.  Although the two last are generally smoked, they must be classed amongst the food adjuncts.  It is of little consequence whether their active principles enter the body by the mouth and saliva or the lungs; their action on the blood and nervous system is the same.

The Extractives, as they are called, comprise a number of bodies of varying nature.  They especially exist in flesh and flesh extracts.  Amongst these are the purins.  They will be treated at greater length hereafter.

Alcohol is to some extent a true food, but its stimulant and other action quite overshadows any food value it may possess.

There are other bodies such as the resins and bitters.  The active principle of Indian hemp is a resin.

There is a great difference of opinion as to the extent to which stimulants may advantageously be used.  It is remarkable that amongst nearly all nations, either alcohol in some form or one of the stronger alkaloids is in common use.  From this fact it is sometimes argued that stimulants must supply a physiological need.  The same method of reasoning will apply with greater force to the use of condiments.  Such conclusions appear to us to be scarcely warranted.