It is well known that individual reactions to the same agents may differ in a very broad range, from very small-size reactions or even absence of any reaction to very great reactions hazardous to life. This attribute of living beings can be best investigated by pharmacological studies. On this aspect van Wijngaarden (1926) performed excellent experiments that explained for the first time the character of this variation. He infused into anesthetized cats very slowly a digitalis solution for toxicity testing — to see how much digitalis should be injected until the heart stopped. He found that the responsiveness to digitalis was normally distributed, which denotes that the responsiveness to digitalis (as also other drugs) could be described by a normal frequency distribution curve, the Gauss curve. According to the normal frequency distribution, a few cats died after a relatively small dose, and some only after a relatively large dose, whereas the majority of cats died at a dose that was similar to the average of all required doses.