It is true indeed that God designed them as remedies for man's ailments, that He prepared beforehand the cures of human ills long previous to the necessity for these cures arising. But this law of mercy was a comprehensive one, and had a two-fold object in view. God in the first place created the plant complete in itself, adapted to its own circumstances and requirements; and in the second place, it is through this perfect adaptability to its own wants that it becomes generally useful in nature, and ministers to the necessities of other created things. It is because the plant heals itself first by the remedy which it grows and produces by its own powers that it becomes a medicine to the animal world, when any members of that world are placed in similar circumstances and exposed to a similar disease. Why, for instance, does the Peruvian bark tree produce the bitter principle in its bark from which we have prepared the valuable medicine called quinine? Is it not because that bitter principle is necessary to preserve the health of the tree itself in the wet, malarial districts where it grows? The Peruvian bark tree grows its own quinine, and administers it to itself, as it were, in order to prevent a disease in itself caused by the marshy places where it is found, similar to fever in the human subject. The willow grows beside rivers and streams which are apt to cause exhalations and breed influences that are noxious to the well-being of the tree. It has therefore developed in its own bark a febrifuge called salicin, which protects it from these noxious influences and maintains its trunk and branches and foliage in vigorous health and beauty. And it is because the quinine is good for the tree itself in malarial places that it is good for the fever which human beings take in such places; and it is because the salicin of the willow guards the tree from the injurious exhalations of marshes and river banks that it is a specific for rheumatism in man, which is produced by the same causes.