PHYSIOLOGICAL RESPONSE IN PLANT AND ANIMAL 741 



of the plant or animal, by inducing the abolition of response. 

 Irritability or molecular responsiveness, therefore, must be 

 regarded not as characteristic of organic substances alone, 

 but as the universal property of matter. In the case of what 

 is commonly known as the living, we have merely higher 

 complexities, with greater instabilities, of molecular structure. 

 External stimulus is here liable to induce greater derange- 

 ment, and the irreversible molecular change known as death 

 takes place the more easily, the more highly organised the 

 complexus may be. Bacteria, for example, will survive con- 

 ditions which would immediately prove fatal to more com- 

 plex organisms. 



In studying the responsive phenomena of living organisms, 

 therefore, we must fix our attention on their molecular aspect, 

 and try to follow out the physico-chemical changes which are 

 consequent on the molecular derangement induced by stimu- 

 lus ; and we are more likely to succeed in obtaining a grow- 

 ing insight into the various phenomena of life, when we 

 approach the subject from this point of view, than when we 

 permit ourselves to evade each difficulty as it arises by 

 referring it to the inexplicable action of a mystical vital 

 force. Physical and mechanical considerations at first appear 

 to us to be inadequate to the explanation of the complex 

 movements of the living machine, just as the similarly com- 

 plex movements of a wind-motor, connected with a hidden 

 electrical apparatus, would at first sight be inexplicable 

 to an inexperienced observer (fig. 278). Let such an 

 observer be brought face to face for the first time with such 

 a windmill. Its movements under the action of wind will 

 arouse his wonder, and this will be increased when some- 

 times even in the absence of wind he sees the vanes revolv- 

 ing still, but now in an opposite direction. He may again 

 notice oscillatory rotations, now in one way and then in the 

 other. Failing to find any rational explanation of these 

 movements in this first stage of his inquiry, he will be 

 driven to attribute them to an unknown power, whose 

 characteristic it is to manifest itself by such erratic actions, 



