Although it is probable that, at some future time, we may be able to make artificially drugs which will be able to produce on the organism any effect which we desire, yet many years must elapse before this can be done, and in the meantime we may possibly find our purpose served, at least to a certain extent, by the use of new remedies of vegetable origin. In our search for such remedies we may be aided by the knowledge that plants which resemble each other in some of their botanical characters, while differing in others, frequently contain principles which exert upon the body actions similar in their general features, but differing in details.

Thus, we find that various species of the genus Rhamnus contain principles which have a general similarity in action, but at the same time present such differences that the action of one species may be uncertain and painful, while that of another is certain and free from discomfort.

In plants which are so much less closely connected as no longer to belong to the same genus, but yet belong to the same natural order, we find differences varying considerably in amount. For example, we sometimes find the action of different genera in one natural order, e.g. Gentianaceae, is as much alike as the action of different species in a genus belonging to another order. At other times we find, as in the Cucurbitaceae and Atropeae, that plants belonging to all the genera in an order have a tendency to produce somewhat similar actions, but these actions vary very considerably in regard both to their intensity and quality. In other cases we find plants which are so closely allied as to belong to the same genus contain active principles which have apparently an entirely opposite action. Thus one species of strychnos will yield strychnine, which causes death by convulsions, while another will yield curara, which kills by paralysing the motor nerves.

But more than this, we find that principles having a very different, or even an antagonistic, action are frequently contained in the same plant; thus from the poppy and from Indian hemp we can obtain morphine and cannabin, which are almost pure narcotics, and thebaine and tetanocannabin, which are almost pure convulsants. From Calabar bean we obtain physostig-mine, which paralyses the spinal cord, and calabarine, which stimulates it so as to produce convulsions. From jaborandi we get pilocarpine, which stimulates the ends of secreting nerves, and jaborine, which paralyses them.

It is thus evident that the action of many drugs will depend upon the proportion in which their active principles are present in them, and it is possible that the proportions may be such that the drug may entirely fail to produce its usual action, as would be the case, for example, if the proportion of jaborine in jabo-randi leaves should be sufficient to neutralise the action of the pilocarpine.

It is just possible, also, that the proportion may occasionally be such as to reverse the usual action of the drug, and the effect of a mixture of alkaloids may sometimes be considerably influenced by the greater susceptibility of the patient to the action of one or other of them.

A great deal of light has been thrown on the relationship to each other of the alkaloids in individual plants or in allied species by Crum-Brown and Fraser's discovery that the addition of alcohol-radicals to alkaloids sometimes completely alters their action; so that methyl-strychnine, e.g., has an action like curara.

It is probable that the active principles in plants are formed by the decomposition of the albuminous matter in their tissues (p. 99), and that the quantity, the quality, and the proportion of different principles present in the plant may vary with the period of growth, and with the conditions under which the plant is grown. Thus hyoscyamus is comparatively inert in the first year of its growth, but it becomes active in the second year; and the common hemp has little or no narcotic power when grown at moderate temperatures, but acquires it when cultivated in a warm climate, as that of India or the Southern States of America.

It is not at all improbable that the active principles of plants may vary even with the time of day, for Sachs has found that a great variation certainly takes place in the amount of starch present in leaves, so much so, indeed, that leaves gathered at evening contain starch in considerable quantities, while it may be almost absent from others gathered before sunrise. The old herbalists were very particular regarding the times at which plants were to be gathered, and it is quite probable that by more attention to such minutiae, we might obtain remedies more certain in their effect.

At the same time, by investigating the physiological action of various plants, we may possibly be able to obtain a series in which the actions vary regularly from one another, so that we can select the one which will best suit our purpose.

A mere knowledge of the names of species, genera, or natural orders is perfectly useless, for the names are liable to be changed at the will of botanists, but a knowledge of the botanical relationships of plants may be a useful indication in our search after new remedies.