THE BEGINNINGS OF LIFE. 271 



differ according to the degree of putrescibility of the 

 solution, the amount of heat to which it has been 

 exposed, and other modifying circumstances. Those 

 which have been produced at the same time are often 

 pretty uniform in siz,e, so that the different dimensions 

 are frequently more marked in different solutions than 

 between Bacteria existing in the same solution. They 

 are, in their most common form, straight, rod-like 

 bodies, varying in length from ^J^nr" to ^W f an 

 inch ; and they generally present a joint or line in the 

 middle, dividing them into two equal parts. Their 

 movements are frequently of a more or less rapid, 

 oscillating, or irregularly-rotating character ; though at 

 other times they may be seen darting from place to 

 place, either directly or in curves of various de- 

 scriptions. All gradations exist, in fact, between 

 movements which suffice at once to stamp them as 

 living things, and mere slow oscillations, the presence 

 of which alone may make us doubtful as to whether 

 we have to do with living or with dead organisms. 



It should be distinctly understood, however, that v 

 such Bacteria as are above described, with all their 

 differences in size, only constitute one variety of the 

 many lower forms of life met with in organic solu- 

 tions. The most varied and diverse forms of these 

 simple organisms exist, and the Bacterium already 

 alluded to is only to be considered as the most con- 

 stant and abundantly represented type. Instead of the 

 rigid, simple, or bi-segmented, staff-like bodies, we may 



