LIVING THINGS AND NON-LIVING THINGS 17 



environment, they are combined in ways that are peculiar to living 

 things. Certain compounds are found in nature only in the bodies of 

 plants and animals. Some of the more common of these are sugars, 

 starches, fats, albumins, certain pigments, and woody and horny sub- 

 stances. These substances are not themselves alive, for we find 

 them also in the dead bodies of organisms. 



Formerly substances that could be obtained only from organisms 

 were called organic and were distinguished from other substances 

 occurring in nature, which were called inorganic. The chemists have 

 succeeded, however, in producing large numbers of compounds that 

 naturally occur only in plants or animals, so that this distinction is not 

 used so much to-day. 



Since the forms and the structures of organisms remain 

 about the same after death, and since probably most of the 

 compounds remain the same, we must find other distinctions 

 between living and non-living. 



26. Growth. The fact of growth is universal for living 

 things. This does not mean that every living body is con- 

 stantly growing ; it means that every organism is capable of 

 growth at some time during its life, or that parts of the body 

 are capable of growth. Yet the crystals of many substances 

 also grow, some of them very rapidly, so that we can actu- 

 ally see them grow. Most of us have seen icicles grow. If 

 by growing we mean becoming larger, then crystals and icicles 

 grow just as truly as caterpillars or babies. What, then, is the 

 real difference between the two kinds of growth ? 



When an icicle becomes larger it does so through the addi- 

 tion of new layers of ice-stuff (water) on the outside. The 

 growth of a crystal proceeds in the same way. A baby, how- 

 ever, does not grow in this manner, (i) The baby grows not 

 by the addition of baby-stuff from the outside, but by the 

 addition of different kinds of stuff such as cow-stuff (milk), 

 or hen-stuff (eggs), or wheat-stuff (bread). (2) The growth 

 material is not added on the surface, but is taken in. (3) The 

 new material does not remain the same kind of stuff, but 

 undergoes chemical changes and becomes at last baby-stuff. 



