i. 3 DEFINITION OF LIFE 3 



shown by using isotopes to discover for how long individual atoms 

 remain in the body; the work of Schoenheimer (1942), which by this 

 means first clearly established the rapidity of the turnover, is a classic 

 of modern biology. 



There are no man-made machines that replace themselves in this 

 way, but in recent years there has been much study of machines that 

 control their own operations. Such work provides us with new analo- 

 gies and new mathematical techniques with which we can analyse 

 the control of living systems (see Yockey and Quastler, 1958). As yet 

 we have no means of grasping the enormously complicated network 

 of activities that constitutes a single life. Throughout this book, how- 

 ever, an attempt will be made to approach that end by use of certain 

 clues to help us to concentrate on significant features, to see the 

 rhythms or patterns common to the lives of the animals, and thus to 

 carry in mind many details. It is possible in this way to bring together 

 information collected by morphologists, geneticists, embryologists, 

 physiologists, biophysicists, and biochemists to give a single view of 

 the life of the organisms concerned. The task is admittedly a hard one 

 and the success achieved only partial. Continually one slips into the 

 discussion of particular structures, substances, or processes, forgetting 

 the whole life. A detail of form or of chemical composition attracts, 

 and thus distracts, attention; perhaps it can hardly be otherwise if we 

 are to describe exactly. But it is surprising how practice improves the 

 powers of selecting and emphasizing those patterns or details of 

 knowledge that are significant for the study of each life as a whole. 



The first difficulty is to force oneself to remember all the time that 

 a living animal or plant system is in a continual state of change. When 

 making any observations, whether by dissection or with the micro- 

 scope, with a test-tube, microelectrode, or respirometer, it is necessary 

 continually to think back to the time when the tissue was active in the 

 living body, and to frame the observation so that it shall reveal some- 

 thing significant of that activity. This means that every biologist 

 must know as much as possible of the life of the whole organism with 

 which he deals ; indeed, something of the whole population from which 

 the specimen was drawn. 



3. Living things tend to preserve themselves 



The clue by which we recognize significant features during any 

 biological study is that living activity tends to ensure the continuance 

 of its own pattern. The processes of life draw materials into the 

 system, organize them there, and then send them out again, all in such 



