June 16, 1911] 



SCIENCE 



929 



this other does not, even though the direction 

 of the rays of light is the same. What is the 

 cause of this difference? Experiment shows 

 it to be the diilerent temperature in the two 

 cases. In another case the difference in mo- 

 tion is found to be due to difference in chem- 

 ical conditions ; or to difference in the amount 

 of food taken, or the like. Many times we find 

 that there are two or more different factors, 

 any one of which will produce the difference 

 in question; or that the observed difference 

 will not result unless two or more determin- 

 ing factors are combined. These are only de- 

 tails of application ; the method throughout is 

 to take two cases differing in a certain re- 

 spect; then to find the (experimental) de- 

 termining cause for this difference. By con- 

 tinuing this process, comparing all possible 

 degrees of difference, the causal analysis may 

 be carried to any desired degree of minuteness 

 — till the smallest perceivable diilerences are 

 reached. The process may be continued back- 

 ward, tracing step by step how the deter- 

 mining differentials for any given case are 

 themselves determined, until we have as full 

 an experimental analysis as we desire, there 

 being no end to the process of analysis, save 

 as practical considerations compel us to stop." 



The investigator may of course not always 

 actually have the two cases present before 

 him; he may not even think of the concrete 

 existence of more than one of the cases, but 

 the rationale of the process, when analyzed, 

 is that which we have set forth.' 



Now, the fundamental principle on which 

 this work of the investigator is based is this : 

 When two cases dijfer in any respect, there 



' The farther work, of comparing the results of 

 this analysis and recording them in fitting gen- 

 eralizations, by which the heap of facts is reduced 

 to an ordered whole, does not concern us here. 



' Thus, when the investigator merely asks : What 

 determines the direction of this movement — the 

 experimental question essentially is. When this 

 specimen moves in a certain direction, while an- 

 other does not, what determines the difference 

 between the two cases? Neglect to analyze prob- 

 lems into this form leads to much of the incon- 

 clusive work and difference of opinion in experi- 

 mental biology. 



will always he found a preceding difference to 

 which the present difference is (experi- 

 mentally) due. This principle is, explicitly or 

 implicitly, constantly present with the experi- 

 menter. If two experiments, supposedly alike, 

 give different results, there must he some pre- 

 ceding difference to account for this. The in- 

 vestigator is so convinced of this that it does 

 not occur to him to doubt it or state it or 

 consciously raise the question at all; he 

 merely sets to work to find what the differ- 

 ence is, and he may spend hours or days or 

 years in his search. This principle is the air 

 the experimenter breathes, the water he drinks 

 and the food he takes. It is what makes him 

 an experimenter. If he should become con- 

 vinced that it does not hold, the logical thing 

 for him to do is to follow the finely consistent 

 example of the sponsor for the kind of vital- 

 ism that asserts that it does not, and drop 

 experimentation to take up philosophy. 



The question whether this principle is cor- 

 rect need not concern us now; what I wish to 

 bring out is the tremendous difference in sci- 

 entific investigation in two fields, in one of 

 which this principle holds (as it is supposed 

 to do in physics and chemistry), while in the 

 other it does not (as in biology, according to 

 this sort of vitalism) . The investigator in the 

 field where it does not hold would be continu- 

 ally in doubt as to what to do. Here are two 

 experiments that result differently. But is it 

 worth while to search for an experimental de- 

 termining factor for this difference? Per- 

 haps there is no such factor — for this is biol- 

 ogy, not physics. The guiding principles are 

 different in the two fields; while we might in 

 physics be certain that an experimental 

 cause could be found for the difference, in 

 biology we can not, for in biology the same 

 configuration may give sometimes one result, 

 sometimes another. This is a difference in 

 principle that would really make it worth 

 while to separate the two sets of sciences in a 

 fundamental way; this would give us a vital- 

 ism that had some practical consequences. 



But what should be the further procedure 

 of the biologist in view of the fact that two 

 complexes absolutely identical in their phys- 



