Il6 TAXONOMY 



and elephants, and were never to be applied to minute plants 

 and animals. There are certain things which are common 

 to plants and animals down to the very simple forms. There 

 is undoubtedly a gradual transition from one form to the 

 other, and consequently no dividing line can be made be- 

 tween the two. The two groups gradually approach each 

 other in their structure and physiological processes, and finally 

 coalesce. 



Conceive then a group of microorganisms at the point 

 where the plant and animal kingdoms unite ; a group which 

 includes animals and plants of the simplest anatomical 

 structure; a group in which differentiation into one or the 

 other of the organic kingdoms is difficult and based upon 

 what little is known about their chemistry, anatomy, and 

 physiology. This group would include certain of the pro- 

 tozoa among the animals and certain of the algae and fungi 

 among the plants. We can make no definite boundary for 

 this group. Haeckel has applied the name " protista " to this 

 group of simple forms of life. 



Bacteria Defined. To this arbitrary group of simple- 

 celled forms the class of microorganisms, which are desig- 

 nated as bacteria, belong. The bacteria are the smallest of 

 all known living organic bodies. The size, of course, varies 

 with the species, but is fairly constant within the limit of the 

 same species. 



It has been decided by careful scientific investigations that 

 the bacteria, taking the whole class of organisms into con- 



