I02 THE ORIGIN OF LIFE 



merited with rendered it extremely difficult to 

 investigate their chemical composition ; but the 

 method of prolonged observation, like the astro- 

 nomical method in matters over which we have 

 no control, enables us to study their structure 

 and behaviour, and to decide the question as to 

 whether they are crystalline or organised and living 

 forms. 



Upon this point, however, it is necessary that 

 the use of the word " crystal " should stand for 

 some definite thing. By a crystal we mean an 

 aggregate of symmetrically arranged groups of 

 molecules. Such aggregates are known to grow 

 by piling up, as it were, one on to another. They 

 grow by accretion, not by assimilation, from their 

 environment. Sachs ^ regarded protoplasm as made 

 up of minute crystals, but he seems to have used 

 the word in, perhaps, a somewhat elastic sense, 

 by thus including protoplasm, a colloidal substance, 

 amongst crystalline bodies. 



If colloidal bodies are aggregates of minute 

 crystals, they are, however, not symmetrically 

 arranged crystals, and the aggregate is not iso- 

 morphous with the constituent crystals, but on 

 the whole amorphous. 



An organism has a structure, a nucleus, and 

 an external boundary or cell-wall, and its vitality 

 may be described as being a continuous process of 

 adjustment between its internal and its external 

 relations. 



1 Physiology of Plants, pp. 206-7. 



