66 ILLINOIS ACADEMY OF SCIENCE 



this they are removed from the blood reaction of human 

 beings, and because they are not fighting this particular 

 blood reaction they gradually lose the power of fighting 

 it. As a consequence, when they are later removed from 

 the cow to the man they have only the powers of cowpox. 



From these facts it will be seen that cowpox and small- 

 pox are two strains of the same thing. One strain has 

 its powers developed to meet the blood reaction of cows 

 but not that of man, while the other has its powers de- 

 veloped to meet the blood reaction of man but not that 

 of cows. Also, that either can be transformed into the 

 other by a course of training designed to develoD one 

 form of power and not the other. 



About 1880 Pasteur discovered that the anthrax bacil- 

 lus cultivated in chicken broth at blood temperature lost 

 its virulence after a few generations and ceased to kill 

 even the mouse. Since then it has been learned that the 

 virulence of many organisms became diminished when 

 they are grown on artificial media. Let us consider what 

 these facts mean. 



When bacteria in small numbers get into the blood they 

 are rapidly killed off. As far as we are at present con- 

 cerned we need not stop to inquire whether this is by 

 phagocytosis, chemiotaxis, or other means, or several 

 means combined. The point here is that animal powers 

 of some kind attack the bacteria, and for these bacteria to 

 withstand these powers and make headway against them, 

 the bacterial powers must be developed by exercise. 



Now, when bacteria are raised on some nonliving sub- 

 stance, as chicken broth, they do not have to struggle for 

 existence against a blood reaction and consequently they 

 lose the power of meeting such reaction. A stallfed bac- 

 terium may be as fat as any other, but he does not de- 

 velop individual powers any more than does a stallfed 

 steer. 



When a person exerts himself by physical efforts, he 

 does certain foot-pounds of work, and foot-pounds of 

 work is something well known in physical science. It is 

 properly called '' energy '^ but is referred to as ''power" 

 when we wish to indicate energy as being expended, or 



