scientist by his contemporaries or succeeding generations depends directly on their confidence in his method. The history of any biological problem will furnish material for a comparison of methods. The present state of the problem will be found to owe its superiority over any earlier stage not simply to the greater number of facts that can be brought to bear upon its solution, but chiefly to more exhaustive methods for the discovery of new facts and interpretation of old ones.
There are comparatively few models that will serve as illustrations of the applied logic of the sciences, or of a sound and complete scientific method. Apart from quantitative exactness, the problems of biology can be given the same rigid application of logical principles as any other science; and in recent years much progress has been made toward giving numerical expression to both facts and laws in biology. The two following examples of scientific method—the one from experimental physiology, and the other from invertebrate morphology—show themselves, on analysis, to be models of vigorous generalization and deductive inference, prompt verification, reinterpretation of old facts, explanations of old contradictions, and removal of old obstacles to a clear understanding of the matters in question.
Fifty years ago Arnold discovered that the iris of the eel's eye contracts, producing contraction of the pupil, on being exposed to light after the eye is cut out of the head, and even when the anterior part with the iris is separated from the posterior part of the eye; but that when the outer or ciliary rim of the iris is cut away no reaction follows. It seemed to be conclusively proved that in the production of the phenomenon light acts directly on the ciliary part of the iris. A few years later Brown-Séquard discovered the same reaction in the frog's eye, and inferred that the light acts directly on the muscle elements of the iris. This inference he left entirely without verification, and even asked himself, without trying to answer, the question why, if light acts directly on the muscle fibers of the iris, it does not act thus on the other muscles of the body. In 1854-'55 Budge, after apparently exhaustive experiments, denied that the pupil of the excised eye contracts when light falls on the iris and not on the retina. In 1859 Müller proved that light acts directly not only on the outer rim of the iris, but more intensely on the inner or pupillary part. After a lull of twenty years in the dispute, Edgren proved that after destruction of the retina there is no reaction at all, and that therefore light does not act directly on the iris. The only fact that remained undisputed in this strife of fifty years was that the pupil of the excised but otherwise unmutilated eye of frogs and eels contracts when the eye is exposed to the light. It was still unproved whether the phenomenon is due to an intraocular reflex