12 OUTLINES OF BACTERIOLOGY 



regions round the North and South Poles are free from bacteria. 

 The manner of their distribution will be more fully dealt with in a 

 later chapter. It will suffice here to state that it is very uneven, 

 for whilst some substances like sewage and milk contain several 

 millions of bacteria per gram, others like sand contain normally a 

 very small number. 



1 4. DO BACTERIA BELONG TO THE VEGETABLE OR TO 

 THE ANIMAL KINGDOM? 



The most common mistake that is made concerning bacteria is that 

 they are all minute animals. This is a mistake, due, probably, to 

 the fact that people find it difficult to associate with the plant life 

 they are familiar with, organisms which are free and motile, character- 

 istics commonly supposed to be monopolies of the animal kingdom. 

 At first even scientists classified bacteria as belonging to the animal 

 kingdom. Although we have no difficulty in allocating a cabbage 

 to the vegetable kingdom, or a dog to the animal kingdom, when we 

 ■come to examine organisms much lower down in the scale of life we 

 find that the characteristics of the animals and the plants tend to 

 approach each other. In fact, there are several organisms of a low 

 scale of organisation that are claimed both by zoologists and by 

 botanists as belonging to their respective kingdoms. Such for instance 

 is Volvox, a description of which will be found in text-books both 

 of zoology and of botany. In the case of such a doubtful form, the 

 judgment is made on the sum total of its characteristics. With regard 

 to bacteria, with the possible exception of the spiral forms, their 

 •characteristics proclaim them to be members of the vegetable kingdom. 

 Their methods of reproduction, of cultivation and of cell-division are 

 so distinctly of the nature of plants, that since the life-history of 

 these bacteria has been accurately known, there has not been any 

 •doubt in the minds of scientists as to their insertion inside the vege- 

 table kingdom. There is some doubt, however, as to the inclusion of 

 the spiral bacteria within the vegetable kingdom ; this applies most 

 ^particularly to Spirochaete, the characteristics of which approximate 

 very closely to organisms that indubitably belong to the animal 

 iingdom. 



