ORGANIC MATTER. 23 



complexity, become progressively less stable. And those 

 most complex compounds into which all these four elements 

 enter, together with small proportions of two other elements 

 that 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 com- 

 ponents, but, being different in their modes of aggregation, 

 possess different properties. This prevalence among them of 

 isomerism and polymerism, shows, in another way, the special 

 fitness of organic substances for undergoing re-distributions. 



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 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 com- 

 promise between fiuidity and solidity. They cannot be re- 

 duced to the unduly mobile conditions of liquid and gas ; and 

 yet they do not assume the unduly fixed conditions usually clia- 

 racterizing solids. The absence of power to unite together in 

 polar arrangement, leaves their atoms 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 atoms, renders them comparatively incapable 

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



