A drug may be defined as a substance which modifies the functions of the body or of some organ without sensibly imparting nourishment.  This action may be one of stimulation or of depression.  A drug is taken for its medicinal action, a food adjunct for its modifying action on food.  It is impossible to give a quite satisfactory definition, or to draw sharp distinctions.  For example, tea, coffee, alcohol and tobacco are sometimes placed in one group, and sometimes in another, according to opinion of their action and the definition of the terms food adjuncts, drugs and poisons.  The difference of grouping often depends upon intensity rather than of kind of action.  If taken frequently and not in quantity sufficient to have a markedly medicinal action, such things are generally called food adjuncts or supplementary foods, although much may be said in favour of a different view.  The volatile oils of mustard, caraway, cloves, etc., are used in medicine; also the alkaloids of coffee and cocoa.  Even honey is used as a mild laxative for infants; that is, as a drug.  The difference between a drug and a poison is one only of degree.  Some of the most esteemed drugs have to be administered in very small quantities, or they cause death; e.g., strychnine and morphine.

Classifications are necessary for methodical study, and for assisting the memory in grasping large numbers of things which can be grouped together.  Classifications, however, are artificial, not due to natural lines of demarkation, but according to man’s knowledge and convenience; hence a group is apt to approach and finally merge into another group, although on first consideration they appeared quite distinct.  The disregard of this often leads to confusion and useless discussions.

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