OF LIVING MATTER 35 



The assimilative processes of Chromacea and many unicel- 

 lular Algae and Phytozoa are only a little more complex, since 

 they build up their substance in the main after the fashion of 

 higher plants, owing to their power of causing mere inorganic 

 compounds to fall into those modes of combination that exist in 

 their own particular kinds of protoplasm. In other of these 

 latter organisms, however, as with Euglenas and some of their 

 allies, organic compounds, existing in their media in a state of solu- 

 tion, are in part made use of in their assimilative processes. 



We have, in these cases, a transition to the assimilative processes 

 carried on by the great majority of Bacteria which are commonly 

 supposed to be wholly unable to build up their protoplasm from 

 inorganic substances, and are dependent upon organic compounds 

 existing in the tissues or secretions of plants and animals — either 

 during their life or after their death — or upon products which have 

 diffused therefrom into the outside world. Much the same kind 

 of processes must also obtain with the multitudes of Protozoa that 

 exist as parasites within the various organs and tissues of almost 

 all kinds of animals — where they find themselves surrounded by 

 assimilable organic compounds which may pass into their substance 

 by osmotic processes, but still have to be coerced in each case into 

 the particular combinations existing in the special variety of proto- 

 plasm entering into the composition of the parasite. In this 

 respect, however, they are only exercising the same kind of power 

 as that which is ever being exerted in every tissue of our own 

 bodies. The blood is the common pabulum carried to all, and 

 from it the different kinds of epithelial cells, of gland cells, of 

 nerve cells, and others abstract the particular materials needful to 

 build up their own special kinds of protoplasm. It is usually 

 assumed that such processes are simpler and easier than those 

 effected by the prototrophic Bacteria, as they are capable of using 

 the simplest inorganic materials out of which to fashion their own 

 peculiar life-stuff. As to the relative simplicity of the two pro- 

 cesses, however, we need not venture to offer an opinion ; though 

 it seems clear that in each case we have to do with mere chemical 

 combinations brought about by contact action — ' catalysis ' as it is 

 termed — concerning which a few words must now be said. 



The remarkable discovery was made by Berzelius in i8io that 

 certain bodies, by their mere presence, were capable of bringing 

 about particular chemical combinations in substances or elements 

 brought into relation with them, without being themselves affected. 



