56 THE USE AND NATURE 



mitted along outgoing nerves to some of the organs of 

 locomotion. 



Heat often acts upon organisms upon all sides alike ; 

 consequently, though it may stimulate their life-processes 

 generally and, in some instances, give rise to movements 

 the latter are not determined in one more than in an- 

 other direction. It is well known to stimulate the ' to- 

 and-fro ' or the gyratory movements of Bacteria, and other 

 of the lowest organisms ; and whilst it also renders more 

 striking and rapid those changes of form which all Amoe- 

 boid Organisms are apt to display, the movements evoked 

 are similarly random and devoid of purpose. 



It is not altogether similar with the influence of Light. 

 This agent almost always, and of necessity, falls more on 

 one side of an organism. Consequently it often suffices 

 to induce movements of the lower forms of life in definite 

 directions, just as it causes similar responsive movements 

 to he executed by the parts of any higher plants which may 

 come fully under its influence. In each case the move- 

 ment, or altered position, is due to some nutritive change 

 that is, to some alteration, whatever its nature, in the 

 activity of the life-processes taking place in the part 

 impressed by the light. So that, whether we have to do 

 with the movement of a Sunflower or with the loco- 

 motions of minute living units, the essential mode of 

 production of the movement is probably similar. 



Of such locomotions of minute living organisms under 

 the influence of light many instances might be cited ; it 

 will suffice, however, to mention the fact that green 

 Zoospores, which may have been uniformly diffused 

 through the water, are very apt, when the vessel containing 

 them is placed near a window, to collect on the surface of 

 the water at the part where most light falls, and the same 

 would hold good also for many Medusae. Minute animal 



