46 ORIGIN OF LOWEST ORGANISMS. 



to argue that the Bacteria which present them, are no 

 more living than are the minute particles of carbon 

 obtained from the flame of a lamp, which may exhibit 

 similar movements. This, however, is a point of view 

 which becomes obviously misleading if too much stress 

 is laid upon it ; and it is more especially so in this 

 case, when those Bacteria which display the most 

 characteristic sign of vitality viz., " spontaneous ' 

 division or reproduction do, at the time, almost 

 always exhibit only the same languid movements. 

 Mobility is, in fact, not an essential characteristic of 

 living Bacteria, whilst the occurrence of the act of repro- 

 duction is the most indubitable sign of their life. It 

 should be remembered, therefore, that any Bacteria 

 which are almost motionless, or which exhibit mere 

 Brownian movements, may be living, whilst those 

 which spontaneously divide and reproduce, are cer- 

 tainly alive whatever may be the kind of movement 

 they present. 



In any particular case, however, can we decide 

 whether Bacteria, that have been submitted to a given 

 temperature, and which exhibit movements resem- 

 bling those known as Brownian, are really dead or 

 living ? If the movements are primary, or dependent 

 upon the inherent molecular activity of the organism 

 itself, they ought, it might be argued, to continue 

 when the molecules of the fluid are at rest ; if, on the 

 other hand, they are mere secondary or communicated 

 movements, impressed upon the organisms as they 



