114 EVOLUTION OF LIVING ORGANISMS. 



purposes, they are really not primarily dependent on 

 specialised sense organs and nervous systems, but are 

 fundamental properties of protoplasm common to all 

 animals and plants. They depend on the universal 

 irritability of protoplasm, based on the instability of the 

 complex compounds of which it is composed, and their 

 capacity for being broken down, modified, or raised up 

 by the action of external stimuli ; or, in physiological 

 language, they depend on changes in the metabolism 

 (p. 11). A protoplasmic connection between the various 

 parts of the mechanism, irritability and conductivity 

 (which is only a special form of irritability), are the only 

 essentials in the process*. 



Tropisms have long been known to occur in plants. 

 Thus shoots being negatively geotropic grow upwards, 

 while the positively geotropic roots grow vertically down- 

 wards. A negatively heliotropic plant grows away from 

 the light, and a positively heliotropic plant towards it, 

 and so on. These results are brought about by differ- 

 ences in the metabolism due to the unequal action of 

 the stimulus on the two sides of the plant structure. 

 In the case of heliotropism, for instance, the side away 

 from the source of light grows quicker than that towards 

 it ; thus curvature is caused, which ceases when the grow- 

 ing apex points directly to the source of light, and both 

 sides are equally affected. 



Exactly the same thing happens with fixed annnaio 

 like zoophytes. Now Loeb, with great ingenuity, has 

 applied the same explanation to the behaviour of free- 

 moving animals, which turn their head towards or away 

 from the source of a stimulus. For instance, in bilater- 

 ally symmetrical heliotropic animals the symmetrical 

 points on the right and left side are equally sensitive to 

 light ; and when light falls more on one side than on the 

 other there will be greater chemical activity on one side 

 than on the other, and the muscles on one side will be 

 more stimulated so as to move the head round towards or 



