Disease and Inununlty 93 



When Pasteur got anthrax virus so weak that 

 it could not survive in the blood of a guinea pig 

 a month old, the powers of the guinea pig were 

 an overload for those germs. But when he came 

 down to something as feeble as a guinea pig a 

 day old, then those weak germs were an overload 

 for that young guinea pig. 



Now the difference between a guinea pig a 

 day old and a guinea pig a month old is a differ- 

 ence in physical powers developed by normal 

 activity in the interval between a day and a 

 month, and not a difference in inheritance. 

 Guinea pigs do not do any inheriting after they 

 are born. A load is measured by the powers 

 necessary to carry it, and as powers increase or 

 decrease in accordance with the extent to which 

 they are exercised, it is evident that what may 

 be an overload at one time may not be an over- 

 load at another. Also, powers may be decreased 

 by simply shutting off the power supply, in 

 which case a load which was less than an over- 

 load might become an overload. For example, 

 a load which a person could carry with ease and 

 not even be aware of its presence might become 

 an overload by reason of an insufficient supply of 



