Disease and Immunity 91 



there is a gain in physical strength but a loss in 

 fertility. In other wide crosses, particularly in 

 plants, the new individuals are both stunted and 

 sterile. In extremely wide crosses in some fishes, 

 there will be a union of germ cells but a break 

 down in segmentation soon afterwards, so that 

 the embryo never is completed. 



The explanation is simple. Each increase in 

 load increases the efforts of the individual to 

 carry the load, and powers are increased in pro- 

 portion to the extent to which they are exercised. 

 This is true up to the point at which a load be- 

 comes an overload, beyond which point each in- 

 crease of load decreases powers and hastens 

 death. Increasing the wideness of the cross in- 

 creases the load upon the cells of the new indi- 

 vidual until the load finally becomes an overload 

 and causes a breakdown and a failure to develop. 



Pasteur found that the anthrax bacillus could 

 be raised on an artificial medium, as bouillon at 

 blood temperature. When so raised the bacilli 

 do not have to fight for life in a hostile blood 

 reaction, and because they do not have to fight 

 they gradually lose their power of fighting, which 

 is their virulence. He used two cultures, a very 



