l8 THE FUNCTIONS OF ANIMALS. 



tinued division, arrangement, and modification of cells. 

 Thus, plants and animals resemble one another in their 

 essential functions, in their cellular structure, and in their 

 development. 



But while there is no absolute distinction between plants 

 and animals, they represent divergent branches of a V-shaped 

 tree of life. It is easy to distinguish extremes, like bird 

 and daisy, less easy to contrast sponge and mushroom, well 

 nigh impossible to decide whether some very simple forms, 

 which Hgeckel called "protists," have a bias towards plants 

 lor towards animals. But the food which most plants absorb 

 is cruder or chemically simpler than that which animals are 

 able to utilise. Thus plants derive the carbon they require 

 from the carbonic acid gas of the air, whereas only a few 

 (green) animals have this power. Almost all animals depend 

 for their carbon supplies on the sugar, starch, and fat already 

 made by other animals, or by plants. As regards nitrogen, 

 most plants derive this from nitrates and the like, absorbed 

 along with water by the roots ; whereas animals obtain 

 their nitrogenous supplies from the complex proteids formed 

 within other organisms. Most plants, therefore, feed at a 

 lower chemical level than do animals, and it is characteristic 

 of them that, in the reduction of carbonic acid, and in the 

 manufacture of starch and proteids, the kinetic energy of 

 sunlight is transformed by the living matter into the 

 potential chemical energy of complex food stuffs. Animals, 

 on the other hand, get their food ready-made ; they take 

 the pounds which plants have, as it were, accumulated in 

 pence, and they spend them. For it is characteristic of 

 animals that they convert the potential chemical energy of 

 food stuffs into the kinetic energy of locomotion and other 

 activities. In short, the great distinction — an average one 

 at best — is that most animals are more active than most 

 plants. Let us condense in tabular summary the time- 

 honoured " distinctions between plants and animals." 



[T.-\1!LE. 



