GREAT STEPS IN ORGANIC EVOLUTION 849 



contrast is between low-feeders and high-feeders, and though it was 

 accentuated by the evolution of chlorophyll, it was in all probability 

 independent of this pigment. Thus there are a number of Bacteria 

 which are able to fix the free nitrogen of the air — a distinctively 

 plant-like performance, yet independent of chlorophyll. The 

 capacity for chemical synthesis implies the making of compounds 

 readily broken up again; but plants do not themselves use nearly 

 all they make. There are movements, as of shoots and roots, of 

 leaves and tendrils, stamens and stigmas, and carried further in 

 cases like Venus's Flytrap and Simdew tentacle; but, on the whole, 

 plants do not expend much of their energy in these active ways. 

 Animals are characteristically users of the explosive carbon com- 

 pounds made by other creatures. Thus, while we may say that 

 "feeding-low" and "feeding-high" is the fundamental difference 

 between plants and animals, the central difference is that typical 

 plants live below their income, typical animals live close up to 



, . ^, • Anabolism /A\ . ^ ^ . , . ' 



theirs. The ratio ^^ , .. — I -^ 1 m the plant is relatively greater 



Katabolism 



a 



than the corresponding ratio ( - 1 in the animal. In the typical 



plant, possessed of chlorophyll, the numerator of this fraction 

 is always much greater than the denominator; in the animal 

 the denominator is always creeping close up to the numerator, the 

 expenditure to the income. Many detailed differences are associ- 

 ated with this central difference: thus the plant -cell becomes 

 characteristically imprisoned in a cell- wall of cellulose, while the 

 animal gains energy by burning down its carbohydrates completely 

 to carbon dioxide and water; and there is not in plants any function 

 closely corresponding to the nitrogenous excretion of animals. Waste 

 products are indeed got rid of in falling leaves and the like, but 

 they also tend to accumulate in the plant body, e.g. in the form 

 of crystals; and may even tend to lower the fire of life. Sometimes, 

 however, they re-enter the nutritive cycle before they become 

 poisonous. 



To the question whether plants or animals appeared first, it may 

 be answered that the early organisms were neither the one nor the 

 other, though surely nearer the plant-like mode of life than the 

 animal-like mode of life, inasmuch as they would need somehow to 

 synthetise proteins from relatively simpler substances not yet 

 organic. When the predatory mode of life was established, organisms 

 devouring other organisms, then animal life definitely began. When 

 the synthetic processes became dominant in the metabolism, then 

 plant life definitely began. Chlorophyll would soon become character- 

 istic of the majority, though it is possible that it may have appeared 

 before there were definite "plants". In any case, it must always be 

 remembered that it is on the photosynthesis achieved by chlorophyll- 



VOL. II I 



