GREAT STEPS IN ORGANIC EVOLUTION 389 



often, even in man, a transition from one line to another, 

 just as in the life-history of the very simplest Protists, which 

 pass through a cycle of phases without accentuating any 

 one. It is significant that we should see the main physio- 

 logical possibilities blocked out so early. 



\Ve must repeat that if we are asked how there could 

 be in the Primordial Organisms all the promise and 

 potentiality of bee-kind, bird-kind, mankind, we cannot an- 

 swer save to say that the question is not rightly put. But 

 what we may ask is how the Primordial Organisms con- 

 tained the promise and potentiality of the next stage in 

 the Systema Naturae, and it is not so difficult to answer 

 that question. 



4. The Divergence of Green Plants. 



One of the early great events was the emergence of green 

 plants, possibly from an Infusorian stock. Either they or 

 their ancestors had built up chlorophyll, which is probably 

 the most important single substance in the world, for it is 

 in association with this green pigment that the sunlight 

 becomes available to living matter as a source of energy in 

 building up organic compounds. The divergence of plants 

 and animals was one of the great cleavages in organic nature, 

 distinguishing those that feed at a low chemical level from 

 those that feed high, the manufacturers of explosives from 

 those that fire them, the savers from the spenders, the pre- 

 dominantly sedentary from the predominantly locomotor, 

 the anabolists from the katabolists, the sleepers from the 

 wakeful, the captives from the free. The contrast between 

 plant and animal is one of the fundamental dichotomies; 

 parallel dichotomies recur many times in the story of evo- 

 lution, on to the phlegmatic, imperturbable, fatalistic, hypo- 



