This section is from the book "A Compend Of The Principles Of Homoeopathy", by William Boericke. Also available from Amazon: A Compend of the Principles of Homoeopathy as Taught by Hahnemann.
The modern method for arriving at a knowledge of drugs is by experimentation on animals, chiefly frogs, rabbits, dogs, cats, etc. But this method is objectionable on account of the difference in structure and physiology of these animals and the vast difference in their susceptibility to the action of medicines. Nux is most poisonous to man, yet pigs can eat it freely; Aconite is fatal to man in a small dose, yet dogs and horses can eat it with impunity. Birds are not susceptible to the action of Opium or Atropin, etc. Again, the dynamic effects of drugs differ among animals. For instance, Ipecac and Tartar Emetic are emetics to men and dogs, but not to rabbits. Such methods of arriving at the crude drug effects may be sufficient to determine the so-called physiological effects of drugs, and the antipathic use of them based thereon, but is wholly inadequate for purposes of Homoeopathy. They have their use, also, in determining the ultimate organic lesions produced by certain drugs, whenever it is desirable to push a proving to such an extent.
* "The soul does not perceive the external or internal physical construction of herbs and roots, but it intuitively perceives their powers and virtues and recognizes at once their signatum. This signature is a certain organic vital activity giving each natural object (in contra-distinction to artificially made objects) a certain similarity with a certain condition produced by disease, and through which health may be restored in specific diseases in the diseased part. This signatum is often expressed, even in the exterior form of things, and by observing that form we may learn something in regard to their interior qualities, even without using our interior sight. We see that the internal character of a man is often expressed in his exterior appearance, even in the manner of his walking, and in the sound of his voice. Likewise the hidden character of things is to a certain extent expressed in their outward forms. As long as man remained in a natural state, he recognized the signatures of things and knew their true character; but the more he diverged from the path of nature and the more his mind became captivated by illusive external appearances, the more this power became lost". - Paracelsus.