578 THE POPULAR SCIENCE MONTHLY. 



quiry with which we investigate all other occurrences in nature. In 

 the light of the germ-theory, disease is a struggle for existence beticeen 

 the parts of the organism and some parasite invading it. From this 

 point of view, diseases become a part of the Darwinian programme of 

 nature. 



The animal body may be compared to a vast colony, consisting as 

 it does of a mass of cells the ultimate elements of life. Each tissue, 

 be it bone, muscle, liver, or brain, is made up of cells of its own kind, 

 peculiar to and characteristic of the tissue. Each cell represents an 

 element living by itself, but capable of continuing its life only by the 

 aid it gets from other cells. By means of the blood-vessels and the 

 nervous system, the different cells of the body are put into a state of 

 mutual connection and dependence. The animal system resembles in 

 this way a republic, in which each citizen depends upon others for 

 protection, subsistence, and the supply of the requisites of daily life. 

 Accustomed as each citizen is to this mutual interdependence, he could 

 not exist without it. Each citizen of this animal colony, each cell, can 

 thrive only as long as the conditions persist to which it is adapted. 

 These conditions comprise the proper supply of food and oxygen, the 

 necessary removal of the waste products formed by the chemical ac- 

 tivity of all parts of the body, the protection against external mechani- 

 cal forces and temperature, as well as a number of minor details. Any 

 interference with these conditions of life impairs the normal activity 

 of the entire body, or, as the case may be, of the individual cells con- 

 cerned. But the animal system possesses the means of resisting 

 damaging influences. Death or inactivity of one or a few citizens does 

 not disable the state. The body is not such a rigid piece of mechan- 

 ism that the breakage of one wheel can arrest the action of the whole. 

 Within certain limits, any damage done to individual groups of cells 

 can be repaired by the compensating powers of the organism. It is 

 only when this compensating faculty fails, when the body can not suc- 

 cessfully resist an unfavorable influence, that a disturbance arises which 

 we call disease. This definition enables us to understand how external 

 violence, improper or insufficient food, poisons, and other unaccustomed 

 influences, can produce disease. But modern research has rendered it 

 likely that the diseases due to such causes are not so numerous as the 

 affections produced by invasion of the body by parasites. 



Of these a few are known to be animals for instance, the trichina, 

 and some worms found in the blood in certain rare diseases. But the 

 bulk of the hosts we have to contend with is of vegetable nature, and 

 belongs to the lowest order of fungi commonly termed bacteria. 



Special names have been given to the different subdivisions of this 

 class of microscopic beings the rod-shaped bacteria being termed 

 bacilli ; the granular specimens, micrococci ; while the rarer forms, of 

 the shape of a screw, are known as spirilla. 



Bacteria surround us from all quarters. The surface of the earth 



