26 Process of Power Changes 



lated into a cow, it will take only about once in 

 ten trials. In fact, the smallpox germ can sur- 

 vive in the cow only when it comes to some cow 

 which, from some cause or other, happens to be 

 in an unusually weak condition at the time. But 

 when this organism has once succeeded in its 

 fight for life in the blood reaction of a weak 

 cow, it has built up its powers by its own efforts 

 in fighting that kind of blood reaction and then 

 can be passed along to and can live in a cow in 

 whose blood it would have died if inoculated 

 there in the first instance. Then it can be passed 

 along from cow to cow in series, and each time 

 it thus passes it further builds up its powers of 

 fighting the blood reaction of cows by exercising 

 such powers. 



While the smallpox germ is thus busy develop- 

 ing its powers of fighting the blood reaction of 

 cows, it is not fighting the blood reaction of man, 

 with the result that if it be inoculated into a man 

 after having passed through a series of ten or 

 twelve cows, it is found to be cowpox and not 

 smallpox. It has lost the power of fighting the 

 blood reaction of man by not exercising that 

 power, and is able to survive only a short time in 



