98 LIFE IN NATUKE. 



it in every detail. An essentially spiral construction 

 is manifested from the lowest rudiments of life, 

 upwards throughout every organ of the highest and 

 most complex animal. The beautifully spiral forms 

 of the branches of many trees, and of the shells 

 which adorn the coast, are striking examples merely 

 of an universal law. But the spiral is the direc- 

 tion which a body moving under resistance ever 

 tends to take, as may be well seen by watching a 

 bubble rising in water, or a moderately heavy body 

 sinking through it. They will rise or sink in mani- 

 festly spiral curves. Growth under resistance is the 

 chief cause of the spiral form assumed by living 

 things. Parts which grow freely show it well ; 

 the horns of animals, or the roots of seeds when 

 made to germinate in water (as shown before in 

 Fig. 9). The expanding tissue, compressed by its- 

 own resisting external coat, wreathes itself into 

 spiral curves. A similar result may be attained 

 artificially by winding a thread around a leaf bud 

 on a tree, so as to impede its expansion ; it will 

 curve itself into a spiral as it grows. 



The formation of the heart is an interesting illus- 



