18 
Though certain forms are grown for utility, those which cause 
consumption, diphtheria, typhoid and kindred diseases, are studied 
chiefly to discov er what conditions will destroy them, or at least 
heir development or physiological ae or 
counteract the injurious effects of the latter. 
r dependence upon special conditions, or upon certain soils, 
lent in the ability of certain 
tion with other species. Thus, the species which produces tet- 
anus, or lock-jaw, cannot live where there is a free access of 
fresh air or oxygen; but if it can associate with itself another 
species, which does consume oxygen, the two can thrive together. 
Upon the other hand, there are cases in which such associa- 
tion is adverse to the welfare of the g This fact has been 
utilized by introducing the plants of es to the systems of 
those suffering from sarcoma, a disease presenting many simi- 
larities to cancer. The latter i been entirely cured in some 
cases, eee benefited in many o 
It should be noted also that toa ecre extent than any other 
class es plants, perhaps, bacteria possess the power of adapting 
hich ma: 
sa Gb ng them to a favorite soil. Thus the lockjaw bacillus, 
ch as it dislikes oxygen, come to live in the lungs them- 
selves, though none of its aes poison can be produced 
