THE MEANING OF EVOLUTION 26& 



the plant body ; or are non-adaptive variations, pure 

 luxuries, so to speak. 



More than this is represented by the structure of 

 the animal body, but we must first of all consider the 

 points of difference between plant and animal re- 

 garded merely as apparatus in which energ^^-trans- 

 formations occur. In the green plant energy is accumu- 

 lated in the form of high potential chemical compounds, 

 but in the animal energy is expended. Inorganic 

 mineral substances are built up by the plant into 

 carbohydrate, proteid, and fat or oil, but in the 

 animal body carbohydrate, proteid, and fat are 

 dissociated into water, carbonic acid, and urea (or 

 some other nitrogenous excretory substance) ; and 

 the urea or other analogous substance is broken down 

 by bacteria into nitrate, water, and carbon dioxide. 

 The metabolic activities of the animal are said to be 

 " analytic " or destructive, while those of the plant 

 are said to be " synthetic " or constructive, but 

 these contrasting terms hardly describe accurately the 

 essential nature of the activities of the two kinds of 

 organisms. What further constitutes " animality " ? 

 It is purposeful mobility, and the energy-transforma- 

 tions that occur are the means whereby this mobility 

 is attained. The plant is essentially immobile, for such 

 movements as the turning of leaves toward the light, 

 the down-growth of roots, the up-growth of stems, the 

 twining of tendrils round supporting objects, and the 

 opening and closing of flowers are only the movements 

 of parts of the plant organism. They are constant, 

 directed responses to external stimuli — real tropisms — 

 and the extension of this kind of response so as to 

 describe in general the movements of animals is only 

 an instance of the insufficient analysis of facts. The 

 movements of the typical green plant are therefore 



