26 GENERAL MORPHOLOGY AND BIOLOGY. 



bodies the important factor is the molecular constitu- 

 tion. These observations have been confirmed by Ali- 

 Cohen, who found that while the vibrio of cholera and the 

 typhoid bacillus were scarcely attracted by chloride of 

 potassium, they were powerfully influenced by potato juice. 

 Further, the filtered products of the growth of many bac- 

 teria have been found to have powerful chemiotactic pro- 

 perties. It is evident that all these observations have a 

 most important bearing on the action of bacteria, though 

 we do not yet know their true significance. Correspond- 

 ing chemiotactic phenomena are shown also by certain 

 animal cells, e.g., leucocytes, to which reference is made 

 below. 



The Parts played by Bacteria in Nature. As has been 

 said, the great function of bacteria is to break up into more 

 simple combinations the complex molecules of the organic 

 substances which form the bodies of plants and animals, 

 or which are excreted by them. In some cases we know 

 some of the stages of degeneration, but in most cases we 

 know only general principles and sometimes only results. In 

 the case of milk, for instance, we know that lactic acid is 

 produced from the lactose by the action of the bacillus acidi 

 lactici and of other bacteria. From urea we know that 

 ammonium carbonate is produced by the micrococcus 

 urese. That the very complicated process of putrefaction 

 is due to bacteria is absolutely proved, for any organic 

 substance can be preserved indefinitely from ordinary 

 putrefaction by the adoption of some method of killing all 

 bacteria present in it, as will be afterwards described. This 

 statement, however, does not exclude the fact that mole- 

 cular changes take place spontaneously in the passing of 

 the organic body from life to death. Many processes not 

 usually referred to as putrefactive are also bacterial in their 

 origin. The souring of milk, already referred to, the 

 becoming rancid of butter, the ripening of cream and of 

 cheese, are all due to bacteria. 



A certain comparatively small number of bacteria have 

 been proved to be the causal agents in some disease 



