S8 THE FOUNDATIONS OF ZOOLOGY 



Look at a young pine tree and examine its mode of growth. 

 No one can doubt that the long, straight, tapering trunk, and the 

 successive circles of branches, uniformly decreasing in length from 

 the spreading base to the pointed crown, serve a useful end ; that 

 the arrangement offers great resistance to storms, exposes a great 

 area of foliage to sun and air, and has other advantages. Now 

 examine the arrangement of the buds. At the tip of the central 

 axis is a terminal bud, pushing straight upwards and building the 

 crown of the tree, and giving off lateral buds which build the 

 branches, and, becoming their terminal buds, leave behind them 

 their own series of lateral buds to repeat the same process. The 

 shape of the tree, so characteristic that it may be identified miles 

 away, is the result of this simple law of growth ; and this itself is, 

 in a certain sense, a result of the mechanical conditions of life. 

 The bud at the top of the crown is the only one which is sym- 

 metrically placed with reference to the sources of light and air and 

 food, and its symmetry is the result; while the unequal distribu- 

 tion of these conditions of growth results in the one-sided develop- 

 ment of the other buds. If the crown of the young pine tree be 

 destroyed by lightning or storm, or by an enemy, a bud that would 

 otherwise have played a subordinate part, may fall heir to its 

 advantages and build up a new crown. If the tree be prostrated 

 by an accident, a new trunk, with its tapering crown, may spring, 

 in time, from a bud far down the trunk. 



From one point of view the shape of the pine tree seems to be 

 the effect of the mechanical conditions under which it grows, for 

 unnatural or exceptional changes in these conditions may be 

 followed by abnormal deviations from the type ; but from another 

 point of view the type of the pine tree is fi.xed by the constitution 

 or inherent tendency of the tree itself, and is independent of 

 external conditions ; for when a pine, a spruce, and a larch grow 

 side by side under the same conditions, each conforms to its own 

 type. The so-called conditions of individual life are stimuli, without 

 which normal growth does not take place, but they are not deter- 

 mining factors, for the change that follows is due to something 

 prior to and independent of the stimulus. 



While it is a matter of familiar experience, in every moment of 

 our lives, that the stimulus under which a vital action takes place 



