THE LAWS OF PROTOPLASM 139 



Were the space available to review the con- 

 ditions that are known to exist in the simpler 

 animals and plants it would be easy to show that 

 there is a large amount of natural resistance in 

 both of these aspects of immunity. But these two 

 do not always exist to the same extent in the same 

 animal, as the following illustrates : It is well 

 known that sheep are the most susceptible of all 

 the mammals to the toxine produced by the germs 

 of the tubercle bacilli, while the guinea-pig is very 

 slightly susceptible; but the guinea-pig is very 

 susceptible to the tubercle bacilli themselves while 

 the sheep is very resistant. Such examples as 

 these warrant the distinction just made that im- 

 munity to the poison and to the germs are two 

 distinct things. 



Given this natural immunity, which is found to 

 be generally distributed among all forms of life, 

 how does it happen that there is any sickness? It 

 is not easy to answer this question in such an out- 

 line treatment as is given in this book, but it may 

 be suggested that to overcome resistance, the 

 germs must be virulent and the host receptive. 

 We are unable to state just what the conditions 

 are that make the host receptive to germs or how 

 to eliminate the condition. Under certain con- 

 ditions and after a given length of time the 

 symptoms of a definite disease appear In the body. 



These symptoms run their course and the body 



