320 BIOLOGY 



whether there is anything like vital force has not yet been 

 solved, and it is by no means certain that it ever will be. 

 If it were possible for scientists to manufacture a cell exhibit- 

 ing the properties of life, the great problem of biology would 

 be settled. This has never been done, and we must leave the 

 question of the meaning of vitality without an answer. It 

 cannot be insisted upon too strongly that, while we may 

 compare the living organism with a machine, it is unlike any 

 other machine. The living machine consists of a number of 

 small independent units called cells, each one of which has 

 its own independent power of growth and reproduction. The 

 whole combination, too, has functions possessed by no other 

 machine. 



Complex and Simple Living Machines. An animal as high 

 in the scale as the frog is evidently an extremely complicated 

 machine. Not only is it made up of a large number of parts, 

 each with a different function, but each of these parts is made 

 up of a number of tissues, each having a different relation to 

 the organ in general; and furthermore, each of these tissues 

 is made up of hundreds, thousands, and perhaps millions of 

 living units, called cells. It seems plausible to think that, if 

 we could get rid of the complexity seen in the frog, we might 

 approach nearer to primitive life. In other words, if we can 

 get at the simplest unit of life we might be able to understand 

 many mysterious phenomena, since we should thus approach 

 life in its simplest form. For this purpose biologists have turned 

 especial attention to the life of the individual cell, since this is 

 the simplest known unit manifesting life. It is clear, however, 

 from the study of cells in Chapter II, that the mysteries of 

 life phenomena are not solved by reducing them to the opera- 

 tions going on inside of the single cell. Although some cells 

 are simpler than the one shown in Figure 9, still it represents 

 practically the simplest form of machinery with which we are 

 acquainted that is capable of carrying on the functions of life. 

 But such a cell itself is a complex machine, and if we study in 



