124 THE BIOLOGY OF THE SEASONS 



may arrange them on a staircase of five steps, or, better 

 still, on a long inclined plane marked by five more or less 

 clear divisions. 



Lowest are those activities which go on without there 

 being any nervous system involved. Such are the most 

 important transformations of matter and energy that we 

 know of in the whole world, namely, those that go on 

 in green leaves, when, with the help of the sunlight shining 

 through a screen of chlorophyll, they form complex sub- 

 stances like the starch of the potato and the gluten of wheat 

 out of the simple elements of earth, water, and air. It 

 cannot be called an industry in the strictest sense, since 

 plants do not form products outside of themselves, but it 

 may be taken as an example of vital activity that goes on 

 without nervous control. We have no warrant for calling 

 it anything but unconscious. There are many internal 

 activities in animals on the same level ; they go on in- 

 dependently of direct control from the central nervous 

 system. It is well known that the turtle's heart will go on 

 beating long after the bulk of the animal has been made 

 into soup. 



On a second level, there are simple reflex actions, which 

 usually require the possession of a definite nervous system, 

 but are not associated with what we call conscious control. 

 The organism does not know that it does them, unless it 

 happens to fix attention on their performance. We 

 touch a hot iron, and without, in the strict sense, willing it, 

 we draw our finger away. A stimulus has travelled up a 

 sensory nerve to the spinal cord, and a stimulus has passed 

 down a motor nerve to the muscles, commanding them to 

 effective action. This is a reflex action of a simple kind, 

 but there are many more elaborate activities and even 

 parts of industries which appear to be compound reflexes. 

 They are, as we say, automatic. If we may judge from 



