ORGANIC MATTER. 25 



which very readily oxidize, have an instability so great that 

 decomposition ensues under ordinary atmospheric conditions. 



Among these elements out of which living bodies are built, 

 there is an unusual tendency to unite in multiples; and so 

 to form groups of products which have the same chemical 

 elements in the same proportions, but, differing in their 

 modes of aggregation, possess different properties. This pre- 

 valence among them of isomerism and polymerism, shows, in 

 another way, the special fitness of organic substances for 

 undergoing re-distributions of their components. 



In those most complex compounds that are instrumental 

 to vital actions, there exists a kind and degree of molecular 

 mobility which constitutes the plastic quality fitting them 

 for organization. Instead of the extreme molecular mobility 

 possessed by three out of the four organic elements in their 

 separate states instead of the diminished, but still great, 

 molecular mobility possessed by their simpler combinations, 

 the gaseous and liquid characters of which unfit them for 

 showing to any extent the process of Evolution instead of 

 the physical properties of their less simple combinations, 

 which, when not made unduly mobile by heat, assume the 

 unduly rigid form of crystals; we have in these colloids, of 

 which organisms are mainly composed, just the required 

 compromise between fluidity and solidity. They cannot be 

 reduced to the unduly mobile conditions of liquid and gas; 

 and yet they do not assume the unduly fixed condition usually 

 characterizing solids. The absence of power to unite together 

 in polar arrangement, leaves their molecules with a certain 

 freedom of relative movement, which makes them sensitive 

 to small forces, and produces plasticity in the aggregates 

 composed of them. 



While the relatively great inertia of these large and com- 

 plex organic molecules renders them comparatively incapable 

 of being set in motion by the ethereal undulations, and so 

 reduced to less coherent forms of aggregation, this same 

 inertia facilitates changes of arrangement among their con- 



