1887] Parasitic Bacteria and their Relation to Saprophytes. 5 
site being subject to all the contingencies which affect other 
forms of life in nature, it may ingraft itself more and more upon 
the system, or it may die out in the course of time. 
While assuming, without any infringement of known biological 
laws, that all parasitic bacteria were derived from saprophytic 
forms, the difference between them is so sharply defined as to 
‘make us stand in awe at the tremendous power of the one class 
when contrasted with the other. Millions of saprophytic bacteria 
may be introduced under the skin or into the blood-vessels of 
animals without any marked disturbance. A single pathogenic 
microbe, by rapid multiplication within the body, may destroy 
life ina day. The power thus acquired by these minutest and 
simplest of living organisms is one of fearful effect upon the most 
highly organized class of animals. It is awar of pigmies against 
giants, which ends with the destruction of either or both opponents. 
If the giant be only a rabbit, it is at least a billion times larger 
than each microbian opponent. If we take the larger animals or 
man, the relation in size between the microbe and its victim dif- 
fers but little from that of the earth and the meteorite falling upon 
its surface. 
The derivation of pathogenic from harmless saprophytes is well 
suggested by three organisms,—those causing Asiatic cholera and 
typhoid in man and so-called cholera among swine. These or- 
-ganisms thrive very well upon various media, indicating that they 
are not necessarily limited to the living body as a habitat. But 
‘the remarkable feature which they have in common is their power 
of spontaneous movement in liquids. During their parasitic life 
this function does not appear to be of any service whatever. The 
bacteria of cholera are restricted to the small intestine, where they 
‘multiply with enormous rapidity. Those of the other diseases 
‘Mentioned are not limited to the intestines, but may be found 
-growing in the blood-vessels of various organs in the form of 
dense colonies or plugs. The motility must be regarded as a 
feature of their saprophytic life which they would lose if a strictly 
parasitic habit were finally adopted. An illustration of a some- 
what different nature is furnished by the Anthrax bacillus, the first 
disease-germ thoroughly studied, which produces such a rapidly 
fatal malady in many of the domesticated animals and in man. 
According to Koch, it is an inhabitant of certain low, marshy 
regions, where it goes through its cycle of growth without enter- 
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