LIFE AS A RESPONSE 135 



and family or racial cares are its chief aims. Light, 

 vibration or pressure, touch, odours, heat, and dryness 

 or wetness, are its chief conditions. Each of these 

 changes with greater or less regularity. The alterna- 

 tion of day and night, of heat and cold, tell upon all 

 living things. The prehension of food involves the 

 sense of contact with bodies of many kinds ; whilst 

 movement involves changes of pressure. Growth and 

 development introduce new factors. The presence of 

 active enemies or of inimical conditions tests the race. 

 For the lowest animal, therefore, some perception of 

 these fundamental conditions, some power of response 

 to their changing quality, some means of meeting 

 emergencies, seems essential. The surprising result is 

 that even the simplest organism gives no hint of any 

 different life than that pursued by the higher ones. 

 All the senses, dim though they be and exercised 

 without organs, are there. The conflict of ages has 

 totally obscured any primitive life. The simplest 

 creatures are old hands, and tell us nothing of what life 

 would be without the power of response. 



But though we are denied insight into the begin- 

 nings of adaptive response, we can still trace its 

 development in the many and complex forms of 

 animal life. 



In fact, life depends upon these fundamental 

 responses to living or nondiving agencies. To us, in 

 whom the enjoyment of the senses is a luxury, there 

 is always a difficulty in appreciating the close, neces- 

 sarily responsive touch in which the simpler organisms 



