120 THE DISEASES OF DOMESTIC ANIMALS. 



ings, by wliicli are meant food, air, water, temperature, cli- 

 mate, etc. 



These two influences liave tended to fasten upon a given species 

 certain characteristics which become transmissible or hereditary. 



The struggle for existence leads to the survival of the fittest. 



We have already said that in all life this struggle existed. It 

 exists not only between all members of the same species, inclusive 

 of man, but also between the cells which make up our organs. In 

 the battle for nourishment, only those cells which are best adapted, 

 from their chemical and physical characteristics, to their specific 

 work, survive ; the others perish. The worn-out are continually 

 replaced by the new. It is so in all life. 



By contagious or infectious diseases we have learned, to our sat- 

 isfaction, that a living organism enters the animal body. Being 

 a li\ang organism, it must naturally seek for the necessary constit- 

 uents to its nourishment within the organism, or else it must die. 

 Where it does not find it, it causes no infection ; with its inability 

 to procure it, the disease caused by it must cease. 



We have learned that the life of the foreign organism is depend- 

 ent on most of the elements of nourishment upon which the life of 

 the animal organism, as a whole, or its individual parts, the cells, 

 depends. The red blood-cells euter into a struggle for existence 

 with the bacteria for that life-gas, oxygen ; the various other cells 

 of the body for the different chemical constituents necessary to their 

 life, the sum total of which constitutes the life of the infected or- 

 ganism. 



In this struggle, one or the other must win. If the cells are the 

 stronger, the bacteria perish, and the animal or individual infected 

 lives. If the bacteria are stronger ; if, added to their parasitic na- 

 ture, they also produce chemical stuffs at enmity with autositic life, 

 they are in a measure supported by an ally in their stiniggle with 

 the cells of the body, and the autosite dies. 



But, with reference to the immunity acquired, we have but one 

 reasonable explanation, which is that, in some unknown way, the 

 cells are enabled to withstand the influences of their parasitic ene- 

 mies ; they gradually acquire a nature which renders them insus- 

 ceptible to further attacks — i. e., they adapt themselves to the influ- 

 ence exerted by the above-named poisons. 



Like the habitual drinker or smoker, the cells are the same as 

 the individual of which they are the component units : they become 

 accustomed to the alcoholic poison. 



So it must be in those cases where immunity follows vaccination 



