156 Changing Plant and Animal Nature 



There is not only the continuous change which is associated 

 with growth and development. There is the significant fact 

 that remaining alive means continuous change in response 

 to changing external conditions. It is this plasticity of the 

 individual whereby the response is more or less adequate for 

 meeting the immediate needs that makes possible adpistmenty 

 or the individual's survival from moment to moment, as dis- 

 tinguished from adaptation which we think of as the means 

 whereby the race or the species continues from generation 

 to generation. 



These processes of adjustment are not so obvious simply 

 because they are too familiar to be noticed. When the erect 

 stem of a plant is bent over and kept in a horizontal or inclined 

 position, the tip will gradually change the direction of its 

 growth and acquire a vertical posture. A reduction in the il- 

 lumination will bring about an acceleration in the growth of a 

 plant's axis. A one-sided illumination will in most green plants 

 bring about a modification in the direction of growth, so that 

 the tip and the upper surfaces of the leaves come to incline 

 toward the source of light. A reduction in the amount of 

 moisture will lead in many plants to a thickening of the cuti- 

 cle or to the formation of hairy epidermis or to the shorten- 

 ing of the main axis. These and many other responses of 

 plants are so well suited to the individual's need for light and 

 for moisture that we cannot but be impressed by their ap- 

 parent purposefulness. 



Adjustment a Continuous Process 



All living things respond more or less adequately to these 

 fluctuations in the immediate surroundings — or they suc- 

 cumb. In our body we find numberless examples of such 

 fluctuating variations from moment to moment. We say, 

 for example, that the normal temperature of the human body 

 is 98.6° F. But this normal means merely the temperature 

 found most frequently among large numbers of people under 

 ordinary conditions. It leaves a considerable range of in- 



