NATURAL IMMUNITY 67 



the constant body temperature of mammals varies 

 considerably for different species. Birds, as a rule, 

 have a higher temperature than mammals, and rep- 

 tiles are " cold-blooded animals." A disease germ, 

 like the tubercle bacillus, for example, which requires 

 for its development a temperature not very different 

 from that of a healthy man, may fail to infect a 

 pigeon because of its comparatively high, or a frog 

 because of its low, temperature. Certain experiments 

 which have been made by bacteriologists give sup- 

 port to this view. This is the explanation offered 

 by Pasteur of the immunity of fowls against anthrax 

 a disease of sheep and cattle ; and in support of 

 this view he showed by experiment that when chick- 

 ens are refrigerated by being immersed in cold 

 water, after inoculation, they are liable to become 

 infected and to die. Again, the composition and 

 especially the reaction of the blood and other body 

 fluids may perhaps be the determining factor. Some 

 germs do not grow readily in an alkaline medium ; 

 and some animals for example, the white rat- 

 have a highly alkaline blood. Experiments made by 

 the German bacteriologist, Behring, seem to show 

 that the natural immunity of the white rat against 

 anthrax infection is lost when the animal is given 

 food which reduces the alkalinity of its blood. It 

 is probable, also, that the presence or absence of 



