192 THE WONDER OF LIFE 



it is more comfortable for it to have its chemical processes 

 in equilibrium, and its two eyes equally illumined. 



Tropisms, then, are obligatory movements which result 

 from a difference in the role of chemical processes on the 

 two sides of the plane of symmetry. Thus we have 

 phototropisms, or obligatory reactions in relation to a 

 light stimulus, the creatures sometimes moving towards it, 

 like some moths, caterpillars, and fishes, which are said 

 to be positively heliotropic ; and sometimes away from 

 it, like earthworms, maggots, and freshwater Planarians, 

 which are said to be negatively heliotropic. In the case of 

 fixed animals, like sedentary worms, the reaction may be 

 simply a bending towards or away from the light. 



Similarly, there are tropisms in relation to gravity 

 (geotropism), in relation to currents or pressures (rheo- 

 tropism), in relation to diffusing chemical substances and 

 odours (chemotropism), in relation to contact with surfaces 

 (thigmotropism), and so on. In all cases the reaction is 

 obligatory and the tendency of the reaction is to secure 

 physiological equilibrium. As we ascend the scale of 

 being, tropisms are often caught up along with more complex 

 activities, but in many of the lower animals they can be 

 studied more or less by themselves. 



Some observations by Davenport Hooker on newly- 

 hatched Loggerhead Turtles illustrate what is meant by 

 an inborn tropism. The babies move away from red, 

 orange, and green, but move towards transparent or 

 opaque blue. It is probable, at any rate, that this helps 

 them to reach the sea, which is their home, though they 

 are born ashore. After entering the water they swim out 

 to sea, perhaps attracted by the darker blue of the deeper 

 water. In a large sand-pit, from which the ocean was 



