236 CHILDHOOD OF ANIMALS 



fifteen times they ceased to take any notice of the disturbance. 

 Even extremely simple creatures which live attached to a surface 

 by a contractile stem and shrink up to their support when they are 

 touched soon fail to respond to a repeated stimulus. The infusorian 

 Vorticella, the polyp Hydra, behave in this way, and a sea-anemone 

 which at first contracts its tentacles when they are touched will 

 very quickly cease to respond. So also single-celled creatures 

 become accustomed to, and may even reverse their response to 

 weak chemical stimulation ; instead of withdrawing from alcohol, 

 they may move towards it. When the chemical tropism is a response 

 to a food substance, it may cease when the animal is satiated, to 

 be resumed later on, and this may mean no more than a change of 

 chemical interaction due to the changed condition of the fully fed 

 organism. When a sea-anemone is fed a certain number of times 

 in succession with pieces of fish or flesh, it will refuse further pieces, 

 so that here there must be a transference to the tentacles of the 

 change produced by the food in the digestive tract. But a more 

 complicated change in reaction has also been obtained by experi-, 

 ments on the sea-anemone, for one that was fed repeatedly on 

 filter-paper soaked in the juice of a crab ejected the paper, and 

 after some experiments would not even swallow it. 



There is no doubt but that the effect of the stimulus can be 

 modified or reversed by repetition in all the simpler cases that have 

 been tried. These results, however, wear off very quickly in the 

 case of single-celled creatures and the simpler animals, and after 

 a lapse of time, which may be measured in minutes, hours or days, 

 the normal responses to the stimulus are resumed. We may sup- 

 pose the protoplasm and the very simple mechanisms of muscle 

 and nerve concerned to recover from the fatigue or strain, or 

 abnormal chemical condition, into which they have been thrown, 

 and on their recovery to be practically, perhaps completely, un- 

 altered. It is different, however, with higher animals, where there 

 seems to be a power of registration of the effects of experience, a 

 registration that becomes not only more complete but more easily 

 available in vertebrates than in other animals, and in the higher 

 vertebrates than in the lower vertebrates. The organic mechanism 

 itself consists of a simple form of sensitive organ to receive the 

 stimulation, nerves to transmit the message to a group of nerve- 

 cells which again directly or indirectly control a group of muscles 

 to carry out the reaction. This mechanism may be permanently 

 altered by experience, but, besides such alteration, the results of 



