INTRODUCTION 5 



because, as we put it in everyday language, the stone cannot 

 " feel." Any force, mechanical, electrical, or heat change, light 

 change, gravity, and so on, that can in any way effect an organism 

 we speak of as a stimulus. The animal, then, has a means of 

 appreciating stimuli, and this is true also of plants. We can test 

 this very readily if we put a living plant near a window, when we 

 shall find that the growing parts turn towards the light, the plant 

 then can appreciate the stimulus of light. A second factor is also 

 involved in the initial experiment, and that is that not merely is 

 the stimulus received, but a reply is made to it, and still further, 

 the reply is generally a fitting one. Here, then, we have had an 

 illustration of a fundamental vital phenomenon, namely, the power 

 of making a response, usually a suitable one, to a stimulus, and we 

 term this attribute Irritability or Sensitivity. 



Any particular living being occupies a characteristic situation 

 or, as we say, habitat, where it is constantly subjected to a stream 

 of typical stimuli reaching it both from within and without. The 

 whole of the stimuli that affects an organism from birth to death 

 we can conveniently include in the one term Environment. It is 

 obvious, on reflection, that the nature of the habitat determines 

 to a large extent the environment. Thus, for example, it is clear 

 that the stimuli affecting an animal like a fish living in the water 

 are different from those reaching an air-dwelling form like a bird. 

 Moreover, while both may have a number of stimuli in common, 

 such as external temperature changes, or internal feelings of hunger, 

 and so on, each is subjected to stimuli peculiar to its habitat, and not 

 playing upon the other, so that each comes under the influence of 

 a different and characteristic environment. .Again, when we 

 consider the response that is to be made to these manifold stimuli, 

 we shall see that in order to live an animal or plant must reply in 

 such a way as to preserve its life in its environment. Hence, this 

 power of irritability is not only a fundamental characteristic of 

 organisms, but within certain limits an absolutely essential one 

 for self-preservation. 



Metabolism. 



It is a matter of common knowledge that living beings 

 require food in order to live, and the nature of this food and the 

 manner in which it is dealt with constitute the second -of the vital 

 phenomena. While an inorganic body, such as a crystal, is able 

 to increase in size or grow, it can only do so when it is provided 

 with a substance that is chemically similar to itself. On the other 

 hand, an organism is able to utilise materials unlike itself for food, 

 and from- them to build up its own substance. This is a capability 



