This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
A great fault - and one which is only too common in the works of experimental pharmacologists - is that of drawing general conclusions from limited data.
One experimenter tries the effect of a drug, let us say tartar emetic, upon rabbits. He finds that they do not vomit, and instead of drawing the only warrantable conclusion, viz. that tartar emetic does not cause vomiting in rabbits, he draws the general one - that tartar emetic does not cause vomiting in animals. Another tries it upon dogs, and he finds they all vomit. Instead of the limited conclusion that tartar emetic makes dogs vomit,
1 Lauder Brunton, 'Action of Nitrite of Amyl on the Circulation,' Journal of Anatomy and Physiology, vol. v. p. 95.
he draws the general conclusion that it makes animals in general vomit. The two observers are equally positive in regard to their facts - each is assured that he himself is right, and that the other is totally wrong. The reason of the discrepancy is simply that the conditions under which the experiments have been performed were different, but the observers have not taken these differences into account when drawing their conclusions. A third observer then comes, perhaps, and by further experiments reconciles the apparently contradictory statements. Thus one experimenter tries the effect of caffeine upon frogs; he finds that it produces rigor mortis in the muscles. Another tries the same drug, and finds no such result. These two observations are completely contradictory, until a third tries the effect of the drug upon two species of frog, and finds that while the muscles of the rana esculenta are but slightly affected, those of the rana temporaria are rendered rigid.1
These apparent contradictions in the results of different observers are exceedingly puzzling to the student, but nothing is more instructive to those who are actually working at the subject.
The utility of apparent exceptions was fully recognised by Claude Bernard, who says: ' In physiological studies we must always carefully note any fact which does not accord with received ideas. It is always from the examination and the discussion of this exceptional fact that a discovery will be made, if there is one to make.'2
1 Schmiedeberg, Arch. f. exper. Path. u. Pharmak., Bd. ii. p. 62.
2 Bernard, Liquides de l'organisme, torn. i. p. 258.