G86 DISTRIBUTION AND HABITAT OF THE BACTERIA. 



also in solutions in which the nutriment is exhausted, 

 and at the highest temperature reached by insolation at 

 the surface of the soil. 



Nevertheless, the effect of these means may he great, 

 and indeed sufficient for the purpose, because oppor- 

 tunities for the sprouting of the spores often present 

 themselves in nature, and they thus assume a vulnerable 

 form; and hence it is chiefly by a constant variation 

 between good nutritive conditions on the one hand, and 

 the absence of water and nutritive materials on the other, 

 that destruction of the various kinds of bacteria and 

 regulation of the bacterial life are brought about. 

 Fate of In the case of those forms of bacteria which excite our 



bacteria. special interest by the fact that they can at times develop 

 in the bodies of the higher living animals, their continued 

 existence in our natural surroundings is rendered more 

 especially difficult, because they are for the most part 

 very particular as to the quality of their nutriment, 

 require a favourable temperature, and are often extremely 

 sensitive to alterations in the nutritive substrata, and to 

 the amount of water present. Further, all the faculta- 

 tive parasites are very readily overgrown by saprophytes, 

 which grow much more quickly under the conditions of 

 life present in our surroundings; the latter therefore 

 withdraw from the former the necessary nutritive 

 materials and also injure them by the products of 

 their tissue change. If, therefore, it is necessary for 

 infective agents to grow for a considerable time under 

 the ordinary conditions, they must evidently have the 

 opportunity of growing in a sort of pure cultivation ; and 

 such an exclusive occupation of the substratum by patho- 

 genic bacteria sometimes occurs on semi-solid nutritive 

 materials, on floating portions of vegetable or animal 

 tissue, &c. In fact the preservation of facultative and 

 obligatory parasites which have grown in this way out- 

 side the human body, or which have grown in the human 

 body, and passed from it into the surrounding world, is 

 a matter of considerable difficulty. They are most easily 

 preserved when spore forms are present, which can per- 

 sist unaltered for a long time in a dry condition, or in 



