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 Ameba, 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 (cel/s 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 protezds, 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 
