GREAT STEPS IN EVOLUTION 79 



ning we find a fertilized egg-cell, which 

 divides and re-divides, the body of the plant 

 or animal being built up by continued divis- 

 ion, arrangement and differentiation of cells. 

 But important as these resemblances be- 

 tween plants and animals are, the divergence 

 is very profound and expresses one of the 

 great cleavages in evolution. 



It came about through the invention of 

 chlorophyll by some Protists a chemical 

 and physiological achievement of the highest 

 magnitude, which made the life of plants 

 possible, and, through them, that of animals 

 and man. In the complex " photo-synthesis " 

 by which plants build up complex carbon- 

 compounds from the raw materials of water, 

 air and earth, chlorophyll plays an indispen- 

 sable part. The still widely prevalent ig- 

 norance of this fundamental process of the 

 living world is perhaps the greatest example 

 of the slowness with which the discoveries 

 of science become generally recognized. 



Most plants derive the carbon they require 

 from the carbon dioxide of the air, while 

 only a few (green) animals have this power; 

 all the others depend for their carbon supplies 

 on the sugar, starch, fat, etc., already made 

 by other animals or by plants. As regards 

 nitrogen, most plants take this from nitrates 

 and the like, absorbed along with water by 



