70 INTRODUCTION TO SCIENCE 



the laboratory. In studying phenomena like 

 hybridization, we are obviously on much surer 

 ground with experiment than with observation 

 in natural conditions. 



Alterations in the conditions of occurrence 

 which it might be difficult or impossible to 

 arrange in Nature can be readily effected in the 

 laboratory. It is thus possible to discover which 

 of the antecedents are causally important. Cattle 

 begin to die of some mysterious epidemic disease; 

 bacteria are found to be abundant in the dead 

 bodies; it is conjectured that the disease is 

 bacterial. Some of the bacteria are peculiar, and 

 it is observed that they occur in all the victims. 

 The hypothesis is made that this particular species 

 of bacterium is responsible for the disease. But 

 since the epoch-making experiments of Koch 

 which showed that Bacillus anthracis is the cause 

 of anthrax (splenic fever, or wool-sorter's disease 

 in man), no one dreams of stopping short of the 

 experimental test. The suspected bacillus is 

 isolated, a pure culture is made, this is injected 

 into a healthy animal, and if the disease ensues 

 the proof is complete. 



Besides furnishing fresh data, an experiment 

 may be of use at a later stage in scientific proce- 

 dure, namely, in putting the hypothesis to the 

 proof; and much of the success of a scientific 

 worker often depends on his ingenuity in think- 



