160 THE NATURE AND ORIGIN OF LIFE 



cations of the colloid state. All this passes unperceived, 

 and yet a moment's reflection is enough to understand that 

 the very fact of numerous living beings existing in this 

 drop of hay infusion and there assimilating and excreting 

 makes it impossible for us to consider identical any two 

 points of the infusion, no matter how close to each other. 



In the impossibility of analysing so heterogeneous a 

 medium, the observer who is slothful or disinclined to 

 reason prefers to conclude that the movement of infusoria 

 is spontaneous. 



Experimentally it is impossible to banish heterogeneous- 

 ness from a medium wherein numerous cells are living ; 

 but we can arrange things so that one factor of movement, 

 chosen beforehand and easy to direct, will act more ener- 

 getically than all the others. We shall not thus obtain a 

 movement produced exclusively by our agent, but the 

 mechanical component which this agent introduces may 

 manifest itself, verifiably and always the same, in all indi- 

 viduals of one and the same species so as to prove, with 

 respect to the agent thus experimented on, the non-inde- 

 pendence of the movements of the unicellular animals 

 under observation. 



Tactisms and Tropisms 



The name of Tactism has been given to a property mani- 

 fested by certain unicellular species the property of being 

 influenced in their movements by a given external agent. 

 When the agent is light we have photo-tactism ; when it is 

 heat, thermo-tactism ; when a chemical substance, chemico- 

 tactism, etc. Tactisms are, therefore, the elements into 

 which we decompose the irritability peculiar to a proto- 

 plasmic species. 



When the sensitive cellular elements are agglomerated in 



