ORGANIC MOVEMENTS 25 



several times, but, if it is not freed from the stimulus, a 

 second type of reaction sets in : the direction of the ciliary 

 movement is reversed. Again without success ; even the 

 third type of reaction, contraction into the tube, is un- 

 successful, and it is only the last kind of motion, swimming 

 away, that definitively frees our animal from the " disliked ' 

 condition. Here quite decidedly the fact that one type 

 of movement has occurred determines the type of the next 

 reaction : the word " trial," though not quite correct even 

 here, seems at least to have a better meaning than if applied 

 to mere movement at random. 



It also might seem to be a typical sequence of reaction 

 types, if to a very weak stimulus our Stentor first answers 

 in its usual original manner, and after that does not react 

 any more : but it seems to me that here we have nothing 

 but the well-known fact of acclimatisation. 



To the last typical class of modifiability of simple motor 

 acts only a few words may be devoted in this connexion. 

 If Stentor, after going through the whole series of possible 

 reactions, is stimulated in exactly the same way once more, 

 it answers with the ultimate reaction at once, supposing 

 the intermediate time has not been very long. And 

 similar features in simple motor actions have been observed 

 in other Protozoa, in Actinians, and some worms. Did these 

 animals acquire any " experience," even of the most simple 

 kind ? And what does " experience " mean in natural 

 science ? A later chapter will have to deal with this most 

 fundamental question. 



Looking back upon the whole of the work done, especially 

 by Jennings, we see that there is nothing very fixed about 

 the most primitive types of animal movement, but some- 



