438 p. MITCHELL 



Watson & Crick [4] to formulate a hypothesis of the structure of deoxyribonucleic 

 acid and to suggest a principle for its duplication [5]. 



There are two main classes into which things fall according to the cause of 

 the persistence of their form. The things in the larger of the two classes, which 

 we may con\-cniently call statids (comprising solids and fluids), owe their form 

 to the physical and chemical bonds between their constituent atoms ; while the 

 smaller class, which we conveniently call fluctids, are formed by a specifically 

 directed flux of certain materials of the environment through the space that they 

 occupy. The shape of a statid is dependent upon the equihbrium between the 

 randomly direaed thermal energy of the atoms and the potential energy of the 

 physical and chemical bonds between them. The shape of the fluctid, on the 

 other hand, is determined by transformations between the potential energy of 

 the component atoms and their spatially directed kinetic energy. The fluctids 

 include the steady-state and open systems that have been considered in many 

 of the recent treatments of the definition of living organisms. The importance 

 of the spatial distribution of the substances in such systems has not, however, 

 generally been considered; but Reiner & Spiegelman [6] pointed out that the 

 entropy increase due to diff"usion may partially compensate for the entropy 

 decrease accompanying the synthesis reactions in hving cells, and more recently 

 Turing [7] reported that, by taking diffusion into account, it could be shown 

 mathematically that stable things could be produced in an initially homogeneous 

 medium by two interacting substances which were generated and destroyed 

 according to simple laws. Turing's model is a perfect fluctid, for its form is not 

 influenced by the type of forces governing the form of statids. This is not, how- 

 ever, the case with most open systems such, for example, as a gas flame, for in 

 this case the form is partly determined by the shape of the burner from which 

 the flame issues. It might, perhaps, be useful to refer to such a system as a 

 fluctoid. The comparison has often been made between flames and living 

 organisms : they owe their similarity to the fact that both are formed by a com- 

 bination of statid and fluctid elements. 



Bertalanffy [8] has pointed out that while an individual hving organism may 

 be compared to 'a hierarchical order of open system which maintains itself in 

 the exchange of components by virtue of its system conditions', the historical 

 character of living organisms does not have its counterpart in the model open 

 systems. This is because Bertalanffy's open systems are effectively perfect 

 fluctids and possess no (statid) chemical memory. A consideration of different 

 kinds of solids shows how certain structures are particularly suited for rephcation 

 and may be capable of stabihzing the individual organism and of causing the 

 historical continuity of the evolutionary process. 



Solids may be divided into classes according to the degree and type of periodi- 

 city of their component atoms or molecules. Schrödinger [2] considered the 

 distinction between periodic and what he called aperiodic solids in biological 

 materials in the following general terms: 'A small molecule might be called "the 

 germ of a sohd." ' Starting from such a small soUd germ, there seem to be two 

 different ways of building up larger and larger associations. One is the com- 

 paratively dull way of repeating the same structure in three dimensions again 



