368 Bacteria , their Nature and Function. 
microbes ; when you eat a slice of bread and butter you therefore 
must swallow as many microbes as there are people in Europe.” 
Here it ought to be stated that a grain of solid matter of London 
sewage contains only a small fraction of this number of microbes. 
But, leaving these silly exaggerations and these grotesque sayings to 
their authors for further improvement, it is nevertheless well esta- 
blished that a considerable number of phenomena in nature are 
intimately associated with bacterial life. The world of bacteria is 
comparable to an unseen flora which, in variety of character, of 
activity, and importance in the economy of nature, compares with 
the visible flora, and in its extension and area of distribution is as 
great as, in some respects greater than, that of the visible vegetable 
and animal kingdom. Though unperceived by the unaided eye, 
this bacterial world forces itself, by its multifarious activity, con- 
tinually on our attention ; it comes into prominence by the vast 
effects, the slow but far-reaching results, which it produces on man, 
animal, and plant, for good and for evil, in life and in death. Some 
of these actions I propose to notice, and it will be seen that while 
there are bacteria whose actions are undesired and not conducive to 
the well-being of man or animals, there are others which are of the 
greatest service both to them and to plants, and are an essentia! 
and integral part in the economy of nature. 
Structure and Life-history of Bacteria. 
I have spoken of the bacterial world as of an unseen flora ; I 
mean by this a part of the vegetable kingdom not perceived by the 
unaided eye, though, nevertheless, it is easily brought to perception, 
by a variety of means. The individuals that constitute the bac- 
terial world are, in fact, of such extremely minute size that only 
by the aid of the microscope can they be seen, their size being often 
less than $ 5 on or guiuo P al 'f of an inch, rarely more than ^Vrr part 
of an inch. They are spoken of as having the character of plants, 
because the elements, like those of a plant, are invested in a sheath 
of cellulose, within which is contained the essential part, the living 
protoplasm, the bacterial individuals being in fact comparable to 
unicellular plants, in which, however, no definite cell nucleus has 
been hitherto demonstrated. It ought, however, to be mentioned 
that various observers have attempted to show, and, by complex 
methods of staining, have succeeded in showing, in some bacterial 
species the existence of parts which resemble, and which are con- 
sidered as comparable to, the nucleus forming an integral part of 
the typical vegetable cell. 
In speaking of bacteria as of plants there are other than mor- 
phological characters which guide us in this designation j bacteria 
resemble plants in this essential, that they possess the power to 
build up, out of simple organic compounds, the most complex sub- 
stances, such as the protoplasm of their own bodies. There are 
known not a few bacterial species which grow and multiply, i.e. 
which build up their highly complex nitrogenous (albuminous) sub- 
