22 GENERAL MORPHOLOGY AND BIOLOGY 



yet know their true significance. Corresponding 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 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 animals, or 

 which are excreted by them. 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 molecular 

 changes takes 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, e.g., the souring 

 of milk, the becoming rancid of butter, etc. Bacterial action 

 also underlies many processes of economic importance, such as 

 the ripening of cream and of cheese, and the curing of tobacco. 



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 circumstances, a suit- 

 able 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 circumstances give rise to 

 disease in either. These are known as saprophytes. They are 

 normally engaged in breaking up dead animal and vegetable 

 matter. Others normally live on or in the bodies of plants and 

 animals and produce disease. These are known as parasitic 

 bacteria. Sometimes an attempt is made to draw a hard-and- 

 fast line between the saprophytes and the parasites, and obligatory 

 saprophytes or parasites are spoken of. This is an erroneous dis- 

 tinction. Some bacteria which are normally saprophytes can pro- 

 duce pathogenic effects (e.g., bacillus cedematis maligni), and it is 

 consistent with our knowledge that the best-known parasites may 

 have been derived from saprophytes. On the other hand, the fact 

 that most bacteria associated with disease processes, and proved 



