234 CHILDHOOD OF ANIMALS 



themselves seriously in their endeavour to escape from a closed 

 chamber, but will be comparatively content if at least one of the 

 sides is protected by light bars. Another set of mammals, of which 

 the small rodents are the best-known instances, at once enter any 

 little hole or aperture and lie placidly in a closed chamber. In 

 higher animals these contact reactions naturally are modified by 

 other factors, such as positive and negative phototropisms, and 

 they are affected by mental factors, but none the less they are 

 linked up with the simplest responses given by single-celled organ- 

 isms. Traces of them survive even in man. In nervous subjects 

 the condition of claustrophobia is well known. Persons affected 

 by it have an almost hysterical dread of closed spaces. They will 

 hesitate to enter a very small room, to go down a narrow alley, to 

 sit in the middle of a row at the theatre, to ride in a closed carriage. 

 Victims of the opposite tendency are said to be affected with agora- 

 phobia. They have a fear of open spaces, will go round two sides 

 of a square rather than cross the open place, are happier in a crowd, 

 prefer a closed carriage, and so on. 



Chemotropisms, or attraction to or repulsions from chemical 

 stimulation, occur not only in free-living cells but in the separate 

 cells and every part of the tissues of higher animals and plants. 

 Some microbes move towards a supply of oxygen, others away from 

 it, so that where such are mixed in a drop of water under the micro- 

 scope they will quickly arrange themselves in a pattern, one set 

 crowded in the centre, the other round the edges where the water 

 is in contact with the air. Weak alcohol repels most free-living 

 cells, weak acid attracts them. Some will move towards solutions 

 of sugar, others towards meat- juice diffusing in the water. One 

 observer found that when he had a number of ciliated animalcula 

 freely swimming in a drop of water under a thin slip of glass, he 

 could kill some of them by applying a hot needle to the surface of 

 the glass cover-slip. The motionless bodies soon became surrounded 

 by a clear space, as if some repelling chemical substance had exuded 

 from the corpses, and he applied to the phenomenon the fanciful 

 name of nekrophobia, fear of the dead. The meeting of the sexual 

 cells, the attraction of food, the senses of taste and of smell are all 

 cases in which chemotropism plays an important part. 



I have given only a few simple instances of these fundamental 

 reactions of living matter. They occur at first without any of the 

 complicated bodily machinery through which they act in the higher 

 forms of life. They are not rigid like the actions and reactions 



