WHAT KINDS OF THINGS ARE LIVING? 17 



appear when the two forms are compared. Yet we consider 

 both types to be living things, and we class both as animals. 



12. Organs; organisms. Now, what similarities can we find 

 among living things in general ? If we consider merely the three 

 examples so far discussed (a seed plant, an insect, a mammal), 

 we find, first of all, that each has a number of distinct parts. 

 If we examine each of these parts more closely, we find that 

 each part is made up of several different kinds of material, 

 arranged in a particular way. Moreover, if we looked into the 

 interiors of the insect, the plant, and the mammal, we should 

 find in each case some more distinct parts. Indeed, most of us 

 already know that our own bodies have such parts as brain, 

 stomach, liver, kidney, heart, and bladder. 



This brings us to a third important fact. Each of these parts 

 is something more than a structural unit, like the many bricks 

 which make up a wall. Each one carries on a particular kind 

 of work or behaves in a particular way in relation to the others 

 or in relation to the plant or animal as a whole. Accordingly 

 we get the idea that each of these parts is an organ, or instru- 

 ment, which performs some special service, or function, in 

 relation to the others or in relation to the whole body. 



Any plant that you know is made up of organs ; any animal 

 that you know is made up of organs. We may therefore call a 

 plant or an animal an organism. To be sure, not all plants or 

 all animals have exactly the same organs ; and, as we shall see, 

 the organs of some plants and animals are not easily distin- 

 guished. Nevertheless, the term organism is a convenient one 

 to use, meaning a living thing. 



13. Chemical characters of organisms. People have long 

 searched the bodies of plants and animals for something that 

 \nll distinguish organisms chemically from non-living things. 

 jVIay there not be some substance or substances peculiar to 

 organisms, something that makes the real difference between 

 living and non-living? 



The moment we start chemical tests with living things we are 

 likely to kill them, or at least to change them in some important 



