2 INTRODUCTION 



beings. Weismann based his theory of heredity on the 

 fundamental conception of the immortality of these 

 unicellular organisms. Excluding accidents, they are 

 immortal they reproduce themselves 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 yet there has been no 

 death, no dissolution ; its protoplasm or living material 

 is still existent in its progeny and is immortal, since this 

 process of reproduction apparently may go on indefinitely. 

 Moreover, the study of the mutability and possible trans- 

 formation of species of micro-organisms is likely 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 

 extensive. These are some of the relations which micro- 

 organisms have with general biology. 



In what may be termed the economy of nature micro- 

 organisms are all-important ; 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 dependent for their 

 existence upon the presence of bacteria in the soil, which 

 break up and render assimilable complex substances used 

 as manures. 



The question of life, animal and vegetable, without 

 bacterial activity is an important and interesting one. 

 It would seem from the experiments of Duclaux 1 that the 

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



