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- 



