INTRODUCTION 3, 
their growth, it is at once evident that any attempt to 
make a hard and fast line separating organic and inor- 
ganic substances must necessarily prove futile. 
There are, however, certain properties peculiar to 
living organisms which may be said definitely to char- 
acterize them, i.e. the power of spontaneous movement, 
nutrition, and reproduction. All of these functions 
are associated directly with that remarkable substance 
protoplasm, most happily designated by Huxley the 
physical basis of life. So far as ordinary chemical and 
physical tests go, the protoplasm of all living organisms 
is much alike; of course this does not imply that pro- 
toplasm is a definite chemical compound such as starch 
or sugar; it is rather to be considered as a mixture of 
excessively complex and unstable substances, more or 
less similar in the elements of which they are made up. 
The constituents of this protoplasm are evidently 
very unstable, as every manifestation of life in the liv- 
ing protoplasm is necessarily bound up with chemical 
changes in its substance. Where the protoplasm is 
present in sufficient quantity to be handled in mass, as 
in those curious organisms, the Slime-moulds, by-pro- 
ducts are usually present which interfere with an accu- 
rate analysis. “ 
The simplest forms of life, like the Bacteria, often 
show little structure beyond a mass of apparently 
homogeneous protoplasm surrounded by a delicate 
membrane, but it is exceedingly doubtful whether this 
extreme simplicity is more than apparent, owing to 
the excessively minute size of these organisms. The 
presence of a nucleus, or at any rate nuclear substance 
in bacteria, is by no means improbable. Among ani- 
