INTRODUCTORY. 5 



Protoplasm has been called the " physical basis of life," because 

 life, whatever that may be, is always associated with it. In fact, 

 some very simple organisms are entirely (or mainly) composed of 

 protoplasm. The Amoeba, for example, one of the lowest of ani- 

 mals, is a minute speck of semifluid protoplasm, which, notwith- 

 standing its simplicity, can and does perform all the functions of 

 life. Generally, however, the existence of protoplasm is more or 

 less hidden by the presence of other substances, formed by or from 

 it, or taken in from the outside. Take, for example, a peeled 

 potato. This is mostly made up of an immense number of micro- 

 scopic compartments {cells or units of structure), each of which 

 contains its modicum of protoplasm. The walls of the compart- 

 ments are made of cellulose, which forms a firm framework, and 

 each of them contains a large number of minute granules of starch. 

 Both cellulose and starch, which form the obvious parts of the 

 potato, are formed from protoplasm. This itself is far less evi- 

 dent, but makes up part of the slime that may be observed on the 

 peeled surface. The vital substance of which we are speaking is, 

 like most very complex compounds, very unstable. After death it 

 breaks down at once, not into elements, but into other simpler 

 compounds, which enter into the composition of its molecule. 

 These simpler compounds are, however, very complex themselves. 

 The most important of them are proteids. composed of a great 

 many atoms of carbon, hydrogen, oxygen, nitrogen, sulphur, and 

 perhaps, in some cases, phosphorus. If, then, the composition of 

 the proteid molecules is so complex, it is obvious that the mole- 

 cule of protoplasm must be far more so. 



It must not be imagined, from what has just been said, that we 

 know enough of protoplasm to regard it as a chemical compound 

 of definite composition. Nor is to be supposed that all the sub- 

 stances found in protoplasm by chemical analysis necessarily help 

 to build up its living molecules. For " protoplasm" appears to 

 consist of an excessively fine network of living organized matter, 

 the meshes of which enclose other substances that are unorganized 

 and not living. 



(2.) Organisms are also characterized by the nature of their 

 external form, which is definite, and bounded by more or less 

 curved surfaces. Non-living matter either has no very particular 

 form {i.e., is amorphous), or else assumes a regular crystalline 

 shape. Crystals are geometrical forms, which are almost always 

 bounded by flat surfaces meeting in sharp edges. 



(3.) Furthermore, organisms exert a great deal of kinetic 

 energy, and this is gained by the breaking-down of the protoplasm 

 into simpler substances A complex chemical molecule is a store 

 of potential energy, and this is changed into the kinetic form by 



