LOGICAL METHOD IN BIOLOGY. 373 



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 illus- 

 trations 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 analy- 

 sis, to be models of vigorous generalization and deductive infer- 

 ence, prompt verification, reinterpretation of old facts, explana- 

 tions 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 an- 

 terior 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-Sdquard dis- 

 covered the same reaction in the frog's eye, and inferred that the 

 light acts directly on the muscle elements of the iris. This infer- 

 ence 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 ex- 

 haustive experiments, denied that the pupil of the excised eye 

 contracts when light falls on the iris and not on the retina. In 

 1859 Miiller 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 



