100 



SCIENCE 



[N. S. Vol. LIV. No. 1388 



ally experimenting and observation is simply 

 taking notes in this great life experiment. 

 Without it the laboratory experiment would 

 lack reality, for it is simply a page torn out 

 of the book of nature with the unknown fac- 

 tors controlled or eliminated. To get at the 

 facts of disease it has been found necessary 

 -to bring experiments as close as possible to 

 the natural phenomena without losing control 

 of the details. 



The necessary association of observation and 

 experiment in interpreting the conditions de- 

 termining disease may be illustrated by the 

 familiar one of the noise and the flash of gun- 

 fire. At a distance we see the flash and then 

 hear the explosion. We might infer that the 

 flash was a cause of the explosion, since it 

 always precedes it. As we approach the scene 

 of operations the noise follows the flash more 

 and more quickly and close by the two reach 

 our senses almost simultaneously. We then 

 are in doubt whether the flash is a cause or 

 merely an accompanying phenomenon. Our 

 static observation at a distance fails to inform 

 us correctly. The experiment of approaching 

 the firing compels us to revise our original no- 

 tion of causation and to make a further study 

 of the entire phenomenon. The gist of the 

 etiological problem is thus to determine what 

 are necessary conditions and what merely sec- 

 ondary phenomena. The experimental method 

 has been of immense service in laying bare 

 the dynamic or causal relation, in other words, 

 the true sequence of events. On the other 

 hand, experiment too far removed from the 

 actualities has frequently led astray when its 

 results were too literally accepted and not con- 

 trolled by observation of the entire phenom- 

 enon. 



In the environment of man and within the 

 human mechanism itself there are many con- 

 ditions operative towards disease. This entire 

 group or sequence of conditions, rather than 

 any single factor in the group or sequence, 

 may be regarded as the cause. If any one of 

 these conditions is neutralized or controlled, 

 the disease may not occur or if in progress 

 it may take another course. Naturally these 

 conditions have different values. They may 



be judged from their accessibility to control, 

 i.e., from a practical standpoint, or from the 

 point of view of the physicist weighing them 

 according to their energy values. We have 

 hardly reached the stage, however, when the 

 conditions favoring disease can be accurately 

 measured. We must still deal with them as 

 entities. Their qualities must engross our at- 

 tention and their quantitative relations remain 

 for a future, more exact medical science to 

 weigh and measure. 



The forces and conditions controlling disease 

 are a mixture of heredity, environment, and 

 parasitism. How can these factors be taken 

 from their natural relations and studied indi- 

 vidually without upsetting the delicate bal- 

 ances of causation? Wliere can we begin to 

 test experimentally the observations we make 

 about natural occurrences? Obviously some 

 very careful surgical operation is necessary in 

 carving out our field of work. In so doing we 

 must realize that we become piece workers 

 tinkering with only a part of nature's mech- 

 anism. Our finished product must be skillfully 

 fitted into the larger mechanism. In attempt- 

 ing to limit our discussion to parasitism as an 

 etiological factor in disease, I realize the difii- 

 culties mentioned. We have not only the dif- 

 ferent categories of environment, heredity 

 and parasitism acting on one another, but 

 within each category we have the animal body 

 reacting with the factors like a chemical proc- 

 ess swinging back and forth towards a state 

 of equilibrimn. Finally, we have in parasitism 

 two living variable organisms capable of ad- 

 justing th(!mselves towards each other in a 

 remarkable degree. 



When, about forty years ago, methods were 

 devised by Robert Koch to make a beginning 

 in the accurate study of bacteria as living 

 agents of disease the contemporary scientific 

 world realized that here were, to all appear- 

 ances, agencies that could be separated from 

 their environment, their life history and ac- 

 tivities subjected to rigid investigation, and 

 their relation to disease opened to demonstra- 

 tion. It is not surprising, therefore, that the 

 bacteriologists of somewhat more than a gen- 

 eration aso, started on their way by these 



