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 study of drug pathogenesy, and its application to the treatment of disease, is furthered by the recognition of different relationships that drugs occupy to each other. Among these, the most apparent, but of least practical value, therapeutically, is the Family relation, or collateral, side relation (congeners), such as belong to the same, or allied botanical family, or chemical group; thus similarity in origin is its claim. In a very broad way, drugs may be divided, according as they belong, to one of the three kingdoms of nature, thus drugs from the animal kingdom, vegetable or mineral. It is not difficult to note certain great features, common to drugs, belonging to one kingdom; but similarity of effects is more marked as different members of a botanical family, or chemical group, are examined. Thus the Ranunculacese family, comprising drugs like Aconite, Pulsatilla, Cimicifuga, etc., show certain symptoms of marked similarity - a family likeness not to be mistaken. This is sometimes so great as to seem identical. For instance, in the case of Ignatia and Nux vomica. Both come from the same order of plants, both contain Strychnia, to the presence of which, undoubtedly, this similarity in effect can be attributed. Now, when this similarity approaches identity of effects, it has been found that they do not follow each other well. For instance, in a given case of stomach disorder, indicating Nux vomica, it is injudicious practice to follow this by Ignatia, on account of its too close resemblance to the symptoms of the former remedy, the results being unfavorable, disturbing rather the normal evolution of the curative influence.