BIOLOGY 



both develop heat and power to do work ; and both give off 

 certain waste matters. The horse, we may say, requires food, 

 but so does the engine ; for coal and water are as necessary 

 for the development of heat and power in the engine, as food 

 and water are for a similar purpose in the horse. 



When we try to state characteristics that will distinguish 

 all plants from all lifeless objects, we find the task still more 

 difficult ; for most plants do not move about from place to 

 place, it is difficult to realize that they give off heat, and they 

 do not give evidence that they have conscious feelings as 

 do the common animals. In spite, however, of these simi- 

 larities, we are usually able to distinguish living from life- 

 less objects at least by the three following characteristics. 



2. Growth of Living Things. In the first place living 

 things use some of the food they eat for growth. No one ever 

 heard of an engine or other lifeless object beginning as a small 

 machine, and then slowly growing larger until it comes to 

 have many times its former weight. 1 Yet this is what hap- 

 pens to all plants and all animals. The average child, for 

 instance, at birth weighs seven to eight pounds ; while a man's 

 weight is over twenty times as great. And if we try to com- 

 pare the weight of an oak tree with that of an acorn from 

 which it started, the amount of increase we find to be enor- 

 mous. 



3. Repair of Living Things. In the second place, parts 

 of a locomotive or of any other lifeless machine by continual 

 use become worn or broken, and the engine must be sent to 

 the machine-shop for repairs. Our bodies, too, are being 

 constantly worn away ; for every time we make a motion of 



* While it is true that icicles and other crystals apparently grow, 

 this kind of growth is brought about wholly by the addition of mate- 

 rial to the outer surface. 



