WHAT KINDS OF THINGS ARE LIVING? 21 



many substances also grow, some of them very rapidly, so that 

 we can actually see them grow. Most of us have seen icicles 

 grow. If by growing we mean simply becoming larger, then 

 crystals and icicles grow just as truly as beets or babies. 

 What, then, is the real difference between the two kinds of 

 growth ? 



When an icicle becomes larger, it does so by adding new 

 layers of ice-stuff (water) on the outside. The growth of a 

 crystal proceeds in the same way. A baby, however, does not 

 grow in this manner. 



1. The baby grows not by the addition of baby stuff from the 

 outside but by the addition of different kinds of material — such 

 as cow material (milk), or hen material (eggs), or wheat stuff 

 (bread). 



2. The growth material is not added on the surface, but is 

 taken in. 



3. The new additions do not remain the same kind of material, 

 but undergo chemical changes and become at last baby stuff. 



4. The growth of the baby goes on not merely by the exten- 

 sion of the surface; it takes place in all parts at once, inside 

 parts as well as outside parts growing. 



These differences between the two kinds of growth may be 

 summarized by saying that the icicle grows by accretion, 

 whereas the baby and other living things grow by assimilation. 

 By accretion we mean the addition of material on the surface. 

 By assimilation we mean the transformation of foreign material 

 into the material of the body, the "making aUke" of stuff that 

 is different. 



17. Movement. Most of the animals that we know are capable 

 of moving about and of moving their parts. Many non-living 

 objects also move, as the clouds and the waves. But these bodies 

 do not move because of anything that takes place inside. We 

 recognize that they are being pushed about by outside forces. 

 In living plants and animals there are movements going on in- 

 side the organism. Some of these inside movements result in 

 the movements which are outwardly visible. 



