10 THE MECHANISM OF LIFE 



— that is, the animal behaves as if it knew very well what it 

 wanted to do and how to do it — but often there is hesitation, and 

 even what we should call caprice, so that we cannot always 

 predict what the creature is going to do. Often, again, an 

 animal appears to behave precisely like ourselves when we " will " 

 to do something — that is, it appears to act spontaneously. 



Now motion in itself is not something that is distinctive of 

 organisms, for the particles of all substances are in a state of 

 continual movement (except at absolute zero of temperature), 

 but this kind of motion is either a fixed, vibratory, or oscillatory 

 one (the regular oscillations or revolutions of the electrons or 

 atoms, for instance); or it is a random movement (as are the 

 molecules of a gas, or the finely divided particles of some sub- 

 stance in suspension in a liquid); or it is such movements as 

 those of the satellites and planets in a solar system, absolutely 

 predictable; or, again, it may be the apparently random move- 

 ments of the " fixed " stars in what has been called their " helter- 

 skelter " flight through space. 



The Meaning of Organic Movement. — Organic movements 

 are, however, different in their tendencies from those that we 

 have just mentioned, though it is not easy to put this difference 

 succinctly and accurately. Perhaps one may say that the 

 movements of plants and animals are, in the first place, growth 

 movements determined by internal causes, and, secondly, they 

 are adaptive movements — that is, useful and purposeful re- 

 sponses on the part of the organism to changes occurring 

 outside itself. Here " useful " and " purposeful " mean that 

 the result of the motion is something to the advantage of the 

 organism. These remarks, it will be noted, apply to both 

 plants and animals. In the case of the former the responses 

 tend to be inevitable responses to simple, natural stimuli, and 

 they can usually be predicted, while in the latter case they are 

 responses to individualised stimuli; they are partly determined 

 by experience, and they cannot usually be predicted. We must 

 not linger on these distinctions here, for we shall return to them 

 in a later chapter. 



Animals, then, tend to exhibit a great variety of adaptive 

 movements, and it is by seeing these that we say that we are 

 looking at an animal; for, obviously, the structure of the latter 

 is almost entirely the means by which the movements can be 

 made. We must now consider what is the nature of the latter. 



