TROPISMS 103 



Take another quality, the emotion of fear. Organ- 

 isms at the bottom of the scale of life exhibit what 

 physiologists call tropisms. Some move towards a 

 supply of free oxygen or towards light, others away 

 from oxygen or from light. These tropisms are 

 subject to organic memory, and the effect of past 

 stimulation may alter, or even reverse, a new stimu- 

 lation, as when a slime-fungus, at first moving away 

 from alcohol, " learns " to move towards it. On the 

 whole the chemotropisms are such that protoplasm 

 shrinks from what is harmful and towards what is 

 helpful. If a crowd of the slipper-shaped animal- 

 culae [Paramecium], distributed more or less evenly 

 in a drop of water under the microscope, be disturbed 

 by kilHng one of them (as may readily be done by 

 pressing a hot needle on part of the cover-slip), the 

 survivors withdraw from the neighbourhood of the 

 corpse, leaving a clear ring round it. A picturesque 

 vitalist has called this phenomenon " nekrophobia," 

 fear of death, but all we know is that the survivors 

 respond to a chemotropism, being repelled by some 

 chemical emanation, or by the absence of a normal 

 emanation, from the corpse. In the lower forms of life 

 the attractions and repulsions are all comparatively 

 simple, and it seems most easy to suppose that they 

 do not act unless within the direct range of actual 

 physical stimuli, mechanical and chemical contacts, 

 waves of heat, light or sound and so forth. But in 

 the ascending scale of Hfe, the range is extended with 

 the presence of more elaborate and highly specialized 

 sense-organs, and these, with their complex organic 

 memories, transform tropisms into a much more 

 subtle and successful mode of appreciating good and 

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