CHAPTER V 



THE BACTERIA 



THE Bacteria, which in these days are familiar, by name 

 at any rate, to everyone, are an extensive group of 

 organisms of the most minute size, and, so far as we 

 know them, of the most simple structure. In their mode 

 of life they bear a general resemblance to Fungi, for, 

 with the rarest exceptions, they are destitute of chloro- 

 phyll, and adapted either to a parasitic or saprophytic 

 existence. They are, however, as we shall see, quite 

 different from any known Fungi in structure and 

 development, though it is not impossible that in some 

 cases a real affinity to Fungi may turn out to exist. 



Both as parasites and as saprophytes, the Bacteria 

 play an enormously important part in the world. 

 Parasitic Bacteria are now known to be the cause of 

 almost all the infectious diseases of man and animals, 

 and in many cases the actual species to which the 

 different diseases are due have been strictly determined. 

 As saprophytes, Bacteria are the great agents of decay 

 of all kinds, owing to the fact that they set up rapid and 

 profound chemical transformations in the organic sub- 

 stances on which they feed. Thus when milk turns 

 sour, or when wine is converted into vinegar, or proteid 

 substances, such as meat, undergo putrefaction, the change 



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