CLASSIFICATION. 15 
Again, i, must be remembered that pathogenic power also de- 
pends, to a greater or less extent, upon the dose injected into an 
animal as compared to its body weight. Some pathogenic organ- 
isms in very minute doses give rise to a fatal infectious malady ; 
others ave only able to overcome the vital resisting power of the 
tissues and fluids of the body when introduced into the circulation, 
or into the subcutaneous tissue or abdominal. cavity, in considerable 
amounts. Some pathogenic bacteria invade the blood; others mul- 
tiply only in certain tissues of the body ; and others again multiply 
in the intestine and by the formation of poisonous products which 
are absorbed show their pathogenic power. 
Another classification of the bacteria relates to the environment 
favorable to their development. Thus we speak of saprophytic and 
parasitic bacteria, or of SAPROPHYTES and PARASITES. 
The saprophytes are such as exist independently of a living host, 
obtaining their supply of nutriment from dead animal or vegetable 
material and from water containing organic and inorganic matters 
in solution, The strict parasites, on the other hand, depend upon 
a living host, in the body of which they multiply, sometimes without 
injury to the animal upon which they depend for their existence, but 
frequently as harmful invaders giving rise to acute or chronic infec- 
tious diseases. Microérganisms which ordinarily lead a saprophy- 
tic existence, but which can also thrive within the body of a living 
animal, are called facultative parasites. Thus the leprosy bacillus, 
which is only found in leprous tissues, is a strict parasite ; while the 
typhoid bacillus, the cholera spirillum, etc., are facultative parasites, 
inasmuch as they are capable of maintaining an independent exist- 
ence, for a time at least, external to the bodies of living animals. 
It seems probable that the pathogenic organisms which are only 
known to us to-day as strict parasites were, at some time in the past, 
saprophytes, which gradually became accustomed to a parasitic 
mode of existence, and, under the changed conditions of their envi- 
ronment, finally lost the power of living in association with other 
saprophytes exposed to variations of temperature, etc. The tubercle 
bacillus, for example, is known to us only as a parasite which has its 
habitat in the lungs, lymphatic glands, etc., of man and of certain 
of the lower animals. But we are able to cultivate it in artificial 
media external to the body ; and it is in accord with modern views 
relating to the development of species to suppose that at some time 
in the past it was able to lead a saprophytic existence. Not to admit 
this forces us to the conclusion that, at some time subsequent to the 
appearance of man and the lower animals in which it is now found 
as a parasite, it was created with its present biological characters, 
which restrict it to a parasitic existence in the bodies of these ani- 
