Hahnemann first made the valuable, practical observation that certain remedies act better when they are given in a certain series. There seems to be an affinity between them. They are not of the same natural family, but of wholly dissimilar origin; but they have marked similarities in action. Such remedies may follow each other well; they point to a deeper and closer relationship than that of mere family, or similarity in origin. Such relationship exists, for example, between China and Calcarea, Pulsatilla and Sepia, Belladonna and Mercurius, Nitric acid and Thuja, Mercurius and Sulphur, etc.