THE MECHANICS OP THE LIVING MACHINE 323 



transforms one type of energy into another. But the living 

 organism possesses additional powers, some of which may be 

 explained some day, while others, like thought and reproduc- 

 tion, appear to be insoluble and place the living organism 

 in a category by itself. If the living organism is a machine, 

 it is also more than a machine, and cannot be compared with 

 any other mechanism in nature. 



WHAT IS LIFE? 



It may be instructive to ask whether we can define life. 

 Although many attempts have been made to give the defini- 

 tion of life, all that can be done is to describe some of its char- 

 acteristics. The primary characteristic of living things is a 

 constant activity, and if we mean anything by the term "life," 

 it must be the guiding force that controls these activities. 

 Our understanding of the word "life" is certainly obscure; 

 but, so far as it means anything, it refers to the engineer that 

 controls the engine, the machinist that directs the activity of 

 the machine, the force that guides the activities of the animals 

 or plants. What this guiding force is we do not know. Some 

 have called it "vital force" and have believed it to be a special 

 force in nature. Others insist that there is no special force in 

 living things, any more than there is in a clock or a watch. 

 Whether there is any force in nature that can properly be called 

 vitality is not yet settled, but it is certain that the phenomenon 

 which we call life is manifested only in those machines which 

 we call animals and plants, and which come from no source 

 except that of previously existing animals and plants. We have 

 no evidence that this force can be created in any way except 

 from life which previously existed. The life force is capable 

 of indefinite growth and expansion, since a fraction of life 

 force, in the form of any single animal, may produce hundreds 

 of thousands of offspring, each of which has the same amount 

 of life force as the original ancestor had. But this life force, 

 although capable of expansion and growth, has, so far as we 



