ANIMAL EVOLUTION 



By AUSTIN H. CLARK 

 Smithsonian Institution 



CREATIONISM, or the idea that 

 living things have at some 

 time in the past been created 

 substantially in the same form 

 as that in which we know them, has at 

 the present time almost wholly given way 

 to a belief in evolution, a doctrine which 

 assumes the gradual development step by 

 step of all the widely varying forms of life 

 from an original form of simple structure. 

 Creationists and evolutionists alike 

 have erred in considering life as a thing 

 apart from the inorganic world, a mani- 

 festation not measurable in terms which 

 otherwise are of universal application. 



THE INORGANIC BASES OF LIFE 



This is far from true. Just what life is 

 we do not know. All living substance is 

 composed of elements found also in inor- 

 ganic substances, though in the bodies of 

 animals and of plants these elements are 

 combined in forms peculiar to living sub- 

 stance, or to the products of living sub- 

 stance. So we may consider life as the 

 ability, confined to groups of certain com- 

 plex carbon compounds, to increase in 

 bulk indefinitely in such a way and in such 

 varied forms as shall enable the increase 

 to take place to the best advantage. 



Increase in bulk, however, and diversity 

 of form are perhaps not entirely disasso- 

 ciated from inorganic antecedents. In 

 igneous magmas and in the minerals 

 formed from them potassium and mag- 

 nesium on the one hand and sodium and 

 iron on the other tend to vary in correla- 

 tion. That is, the igneous rocks and 



minerals which are high in potassium 

 contain much magnesium and but little 

 iron, while if the rock or mineral be 

 dominantly sodic, iron will be high and 

 magnesium low, if these be present. 



This relationship between these pairs 

 of elements appears to be carried over into 

 the organic world, magnesium and potas- 

 sium being essential to plant metabolism, 

 the other pair being of minor importance, 

 while iron and sodium are necessary for 

 animal metabolism, magnesium and potas- 

 sium very much less so. 



In addition to this it has been pointed 

 out that all organic forms may be inter- 

 preted as falling under one or other of the 

 six systems of crystallization. All ani- 

 mals, the great majority of the protozoans 

 and the early stages of the other types, are 

 ultimately reducible to the isometric 

 system, with three equal axes at right 

 angles to each other, and throughout the 

 animal kingdom there is a marked tend- 

 ency for the adults of the more complex 

 types to revert to this simple form. 



Plants, on the other hand, are funda- 

 mentally reducible to the orthorhombic 

 system, with three unequal axes at right 

 angles to each other, or to the tetragonal 

 system, with three axes at right angles to 

 each other, two equal and the third longer 

 or shorter. As a concrete example, the 

 mints (Menthaceas) may be said to have 

 orthorhombic leaves and a tetragonal 

 stem. 



Sodium and iron and the isometric 

 tendency, potassium and magnesium and 

 the orthorhombic or tetragonal tendency, 



52-3 



QUAK. BBV. BIOL., VOL. Ill, NO. 4 



