Spirazines 9 



usually formed according to some simple geo- 

 metric figure, do not contain chromosomes, centro- 

 somes, or other self-perpetuating bodies, and 

 never conjugate. 



The existence of a definite gap in size between 

 the smallest nucleated cells and the largest bac- 

 teria indicates that bacteria represent a distinctly 

 different form of life from nucleated cells, and 

 when we consider the extreme simplicity of their 

 forms and the homogeneity of their internal struc- 

 tures we feel inclined to believe that they consti- 

 tute the most primitive class of living organisms. 

 Even the largest of them are not far above the 

 limit of microscopic vision, and there is every 

 reason to believe that there are innumerable spe- 

 cies, similar in form and constitution to their 

 larger representatives, which are too small for the 

 microscope to reveal. 



In order to avoid a confusion of issues at the 

 outset, we shall for the present confine our atten- 

 tion as much as possible to the bacteria, because 

 these exhibit life processes in their simplest form. 

 The problem of explaining life does not require 

 that we should explain the entire process of evo- 

 lution, but only those processes and character- 

 istics which are common to all forms of life and 

 which must have been exhibited by the most primi- 

 tive form of living matter as it first appeared on 

 this earth. 



