2 A MANUAL OF BACTERIOLOGY 



unicellular organisms. Excluding accidents, they are 

 immortal they reproduce by a process of simple 

 division, an individual dividing, and two daughter 

 forms taking the place of the original parent one, and 

 although the parent has disappeared there has been no 

 death in the ordinary sense, for its protoplasm is continued 

 in its progeny and is immortal, since this process of repro- 

 duction apparently may go on indefinitely. Likewise, a 

 study of the variation, mutations and possible trans- 

 formations of species of micro-organisms may be expected 

 to throw light on the theory of evolution. Organisms 

 such as bacteria multiply so rapidly that fifty or sixty 

 generations may develop in thirty hours, a number 

 which would take years to attain if even the most rapid 

 breeder among mammals were the subject of experiment, 

 and as they occur in vast numbers the opportunity for 

 variation is unlimited. These are some of the relations 

 which micro-organisms have with general biology. 



Micro-organisms are all-important, in what may be 

 termed the economy of nature ; without them there 

 would be no putrefaction, no decay, and the dead remains 

 of animal and vegetable life would accumulate and 

 encumber the earth, which would become barren for the 

 want of the organic matter originally derived from it, but 

 of which there was no return. In fact the higher plants, 

 and indirectly, therefore, animals also, are largely depen- 

 dent for their existence upon the presence in the soil of 

 bacteria, which fix atmospheric nitrogen and break up 

 and render assimilable complex substances used as 

 manures. 



Whether animal or vegetable life is possible without 

 bacterial activity is an important and interesting question. 

 It would seem from the experiments of Duclaux l that 

 in ordinary circumstances the higher plants are unable to 



1 Comp. Rend., t. 100, p. 66. 



