2 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



additional and distinctive properties. These are all 

 connected with the endowment of life. But we know 

 nothing of life except by its properties. The same, 

 however, is true of all other forces of Nature. We 

 know not what they are, but only what they do, how 

 they behave. Life, like all other forces of Nature, is a 

 specific form of energy characterized by a peculiar group 

 of phenomena, the subject-matter of a distinct science 

 — biology. What, then, are the peculiar phenomena 

 of life ? 



i. Organization.— All living things are organized — 

 i. e., consist of different parts having different functions 

 and all co-operating for one given end, the conserva- 

 tion of the organism. It may at first seem that by this 

 definition a machine, say a steam engine, is living; but 

 there is this essential difference : the steam engine re- 

 quires an external force determining the co-operation, 

 while in living things the co-operation is self-determined. 

 2. Cellular Structure. — All living things are wholly 

 made up of cells, as completely as a brick building is 

 made up of bricks. Now there is an unorganized cellular 

 structure also — such, for example, as in foam, in soap- 

 suds, in vesicular lava, etc. But there is again this es- 

 sential difference : in unorganized cellular structure 

 there is simply a homogeneous mass full of hollow 

 spaces, while, on the contrary, in organized cellular 

 structure each cell is a living entity — the organized body 

 is a community of living cells. 



3- Growth. — All living things grow. But do not 

 nonliving things grow also ? Take a saturated solution 

 of sugar, or alum, or salt. By evaporation crystals are 

 formed. Small at first, they grow from hour to hour, 

 from day to day, until they become as large as a walnut 

 or even as one's fist. But again, we find essential dif- 

 ferences in the kind of growth : i. The growth of the 



