322 ORGANISATION IN SPACE AND TIME 



can only exist so long as it carries out an unending succession 

 of multitudinous biochemical processes at a great speed, 

 which together make up its metabolism. Thus it is only 

 necessary for these processes to be suspended or radically 

 changed for the protoplasmic system itself to be destroyed. 

 Its continued existence, the maintenance of its form, is 

 associated not with immutability or rest but with continual 

 motion. Thus protoplasm is not a static but a ' stationary ' or 

 flowing system. 



This characteristic property of living things was already 

 recognised among the ancient Greeks by the great dialec- 

 tician Heraclitus^^ who taught that our bodies flow like 

 streams ; the material in them is renewed like water in a 

 river. In fact a river or a simple stream of water flowing 

 from a tap enables us to understand, in their simplest form, 

 a number of essential features of the organisation of irrevers- 

 ible or open systems, of which living protoplasm is a particu- 

 lar example. If the tap is not fully open and the pressure 

 in the water system remains constant, the stream of water 

 issuing from the tap will stay almost the same shape, as 

 though it had been congealed. We know, however, that this 

 shape is nothing but the visible manifestation of an unending 

 flow of particles of water which continually enter and leave 

 the system at a particular rate. The very existence of such 

 a system depends on the fact that a constant succession of 

 new molecules of water is passing through it at a steady rate 

 the whole time. If the flow is interrupted the stream ceases 

 to exist as such. 



In an analogous way the organisation of protoplasm is 

 based on a stationary state by virtue of the fact that the living 

 organism is constantly exchanging material and energy with 

 the medium which surrounds it ; that within it a series of 

 irreversible co-ordinated reactions are being carried out at a 

 definite rate, as a result of which substances which enter the 

 organism from the outside medium undergo a series of trans- 

 formations within it and the products of their decomposition 

 are again liberated into the outside medium. 



