64 The Outline of Science 



FIRST GREAT STEPS IN EVOLUTION 



THE FIRST PLANTS THE FIRST ANIMALS BEGINNINGS OF 

 BODIES EVOLUTION OF SEX BEGINNING OF NATURAL 



DEATH 



The Contrast between Plants and Animals 



However it may have come about, there is no doubt at all 

 that one of the first great steps in Organic Evolution was the fork- 

 ing of the genealogical tree into Plants and Animals the most 

 important parting of the ways in the whole history of Nature. 



Typical plants have chlorophyll; they are able to feed at a 

 low chemical level on air, water, and salts, using the energy of the 

 sunlight in their photosynthesis. They have their cells boxed in 

 by cellulose walls, so that their opportunities for motility are 

 greatly restricted. They manufacture much more nutritive 

 material than they need, and live far below their income. They 

 have no ready way of getting rid of any nitrogenous waste matter 

 that they may form, and this probably helps to keep them 

 sluggish. 



Animals, on the other hand, feed at a high chemical level, on 

 the carbohydrates (e.g. starch and sugar), fats, and proteins 

 (e.g. gluten, albumin, casein) which are manufactured by other 

 animals, or to begin with, by plants. Their cells have not cellu- 

 lose walls, nor in most cases much wall of any kind, and motility 

 in the majority is unrestricted. Animals live much more nearly 

 up to their income. If we could make for an animal and a plant 

 of equal weight two fractions showing the ratio of the upbuilding, 

 constructive, chemical processes to the down-breaking, disruptive, 

 chemical processes that go on in their respective bodies, the ratio 

 for the plant would be much greater than the corresponding ratio 

 for the animal. In other words, animals take the munitions which 

 plants laboriously manufacture and explode them in locomotion 



