4 Parasitic Bacteria and their Relation to Saprophytes. (Jan. 
eases as anthrax, typhoid, glanders, cholera, etc., multiply 
readily in organic infusions in milk, even in drinking-water, for a 
variable period of time. They grow luxuriantly upon the cut 
surface of a boiled potato, which is a purely vegetable product. 
Bacteria of this kind are without doubt closely related to the 
numberless forms living in the soil and water, and drifted about, 
in a dried state, with currents of air. Yet they differ in some 
physiological function, some chemical power, which enables one 
group to destroy animal life, while the other is itself destroyed 
as soon as it enters the animal body. There are other parasitic 
bacteria which are much more fastidious in their choice of a su 
sistence outside of the body, which shun the boiled potato and 
require conditions approximating those found in the animal 
organism. The bacillus of tuberculosis flourishes only on blood- ' 
serum at the temperature of the body, and the gonococcus, 
according to Bumm, seems to prefer human blood to that of the 
lower animals. 
Finally, there are parasitic forms only known to us from a 
microscopic examination of the tissues which they infest, such 
as the microbe of leprosy, and perhaps of syphilis. Cultivation 
upon nutrient substances has not yet succeeded. We must there- 
fore infer that these forms have become so thoroughly adapted to 
a life in the tissues of the living body that the conditions there 
prevailing cannot be realized sufficiently in artificial culture to 
induce multiplication. 
These facts explain why many pathogenic bacteria can be cul- 
tivated,— grown at will in tubes containing appropriate media; 
‘we simply make use of their capacity for living and multiplying 
upon dead matter, a capacity ancestral in its origin, and suggest- 
ing that all: pathogenic bacteria were derived by a process of 
_ natural selection from the innumerable harmless species every- 
where peopling the air, the soil, and the water. How the para- 
sitic nature of these bodies was acquired gives ample scope for 
speculation, as nothing definite is known. To me it seems most 
reasonable to suppose that many of the bacteria now known to’ 
cause disease acquired: certain physiological properties in their 
natural habitat, preii 5 in warm doantes — ow ac- 
cidentally Me be 
ge was added to the ras ritance c 
