124 



host that will enable them to continue their life-history. 

 When they touch the mollusc they work their way into it and 

 exhibit a remarkable succession of multiplications and meta- 

 morphoses. The point is that a minute, brainless creature 

 responds at once to the one stimulus which will enable it 

 to continue its life. 



These instances have been taken from different levels: 

 the swallow is very intelligent and yet instinctive, the bee 

 is very instinctive and yet intelligent, the larval mussel has 

 just the beginnings of a nervous system, the larval fluke 

 has none. Our point is that we can find objectively analo- 

 gous kinds of behaviour at all levels of nervous organisation, 

 and that we have to do with a general capacity of living 

 creatures the capacity of enregistering past experiences 

 and experiments, either individual or racial, so that present 

 behaviour is influenced by them in very specific ways. 

 There are several characteristic features in behaviour which 

 appear to be beyond all mechanical description. The be- 

 haviour is made up of a succession of acts which are cor- 

 related in a particular sequence. At any one moment there 

 are chemical and physical processes going on, about which 

 ! we know or may know a good deal, but it is the bond of 

 union that eludes the chemist and physicist. To take items 

 in the process and reduce them (as far as we can) -to physi- 

 cal and chemical common denominators is interesting in its 

 way, and for certain purposes useful, but it does not make 

 any clearer the interlinking, the co-ordination, of all the 

 items in a piece of behaviour. When we consider the larval 

 liver-fluke arrested by contact with the fresh-water snail 

 with a particular species of water-snail, or the larval mussel 

 arrested by the proximity of a minnow, or a stickleback, or, 

 it may be, by one and only one particular species of fish, 



