22 GENERAL MORPHOLOGY AND BIOLOGY. 



many bacteria have been found to have powerful chemiotactic 

 properties. 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. Corresponding chemio- 

 tactic 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 chief effect of bacterial action in nature is to break up into 

 more simple combinations the complex molecules of the organic 

 substances which form the bodies of plants and aiiimals, or 

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

 the stages of disintegration, 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, and that from urea ammonium carbonate is 

 produced by the micrococcus ureae. That the very compli- 

 cated 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 molecular 

 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 processes 

 occurring in man, animals, and plants. This means that the 

 fluids and tissues of living bodies are, under certain circum- 

 stances, a suitable pabulum for the bacteria involved. The 

 effects of the action of these bacteria are analogous to those 

 taking place in the action of the same or other bacteria on dead 

 animal or vegetable matter. The complex organic molecules 

 are broken up into simpler products. We shall study these 

 processes more in detail later. Meantime we may note that 

 the disease-producing effects of bacteria form the basis of 

 another biological division of the group. Some bacteria are 

 harmless to animals and plants, and apparently under no 



