194 ANIMAL BEHAVIOUR 



One of the criteria of organisms is the power of retention 

 or registration which eventually finds remarkable expression 

 in human memory, and the general view we wish to suggest, 

 as a clue to the maze of animal behaviour, is that there 

 has been at level after level a process of automatisation or 

 organisation, which makes for economy of time and energy, 

 and also, if it does not go too far, leaves the organism free 

 for experiment and initiative. So in established reactions, 

 in reflex actions, and in tropisms we see enregistrations 

 which are, in a way, off the main line of advance. 



Besides established reactions, reflex actions, and tropisms 

 there are rhythmic activities adjusted to external periodici- 

 ties, such as change of position in shore animals when the 

 tide goes out or comes in. That these may be more than 

 tropisms is shown by cases where the rhythm is so engrained 

 in the creature's constitution that it persists for a time in 

 periodic expression even when the external stimulus has 

 ceased. The interesting green worms called Convolutas, 

 well-known on some flat beaches, such as that of Roscoff, 

 come up to the surface of the sand when the tide goes out, 

 and retreat again when the tide comes in. Bohn has found 

 that they will continue doing this for a couple of weeks 

 in an aquarium away from the sea. Similarly, some hermit- 

 crabs which make for the light at high tide and away from 

 the light at low tide have been observed doing this for some 

 time at the proper hours in a tideless aquarium. 



It is a question for expert discussion and further experi- 

 ment how far the conception of tropisms will carry us as 

 an interpretation of the ways of the lower animals. Loeb 

 believes it will cover most cases; Jennings thinks its scope 

 is narrowly limited. Just as tropisms differ from ordinary 

 reflexes in being usually adjustments of the animal as a 



