218 



LIVING PLANTS 



Various 



physiological 



distinctions 



Cuvier's first proposition. Sedgwick and 

 Wilson in their "Biology" (1886), find the 

 sole characteristic of animals to be depend- 

 ence upon proteid food. Von Siebold in a 

 dissertation upon this subject published in 

 1844, believed he had found a criterion in the 

 contractility of tissues. Some have brought 

 forward the chlorophyll function, or the pro- 

 duction of starch, cellulose, etc. The latest 

 suggestion is probabl}^ that of Conway Mac- 

 Millan (1895), who sees a fundamental dif- 

 ference between animals and plants in the 

 dynamic, energy-liberating nature of the for- 

 mer, expressed morphologically in cephaliza- 

 tion, and in the static, energy -fixing nature 

 of the latter, expressed morphologically in 

 sporophytization. 



Whatever the characters that are selected 

 for our crucial test, it is evident that the}' 

 must apply equally to the highest and most 

 complex forms, to the simplest unicellular 

 forms, and to all intermediate grades. What 

 structure or structures are there so universal, 

 so indispensable to the simplest and to the 

 most complex, to animal and plant alike, from 

 which characters can be drawn of universal 

 application ? The least complex organisms 

 are likely- to furnish most readily the clue to 

 the answer. Setting aside physiological con- 



