DIFFERENTIAL GROWTH 1 47 



faculties for selective combination and segregation of the molecular 

 constituents on the one hand; and the marginal reference frames of 

 surfaces on the other, which, by their power of adsorbing and orienting 

 the mobile elements, force them into a supraelemental collective order. 

 It seems impossible to account for cellular organization otherwise than 

 by such a dual concept. Its essence can be summarized as follows : 

 Manifest cell organization results from the response of organized ele- 

 ments to fields of organized (i.e. non-random) physical and chemical 

 conditions, here tentatively identified with conditions prevailing along 

 interfaces. 



The nearest simile to this type of organization is found in the be- 

 havior of human or animal populations, which likewise consists of the 

 reaction of organized individuals to an organized environment. In both 

 cases, the order is one of ecological conditions. Thus, cell biology, in 

 molecular terms, becomes not merely molecular physics and chemistry 

 of the cellular constituents, but molecular ecology (54), a science not of 

 molecular individuals or pairs, but of molecular populations and their 

 existential conditions. 



Some day we may hope to have a fairly complete list of the chemical 

 compounds present in any given cell, from simple elements to the highly 

 complex organic macromolecules, and thus develop a taxonomic cata- 

 logue of molecular species. But just as a museum collection of animals 

 is never lifelike unless it shows them in their natural environment and 

 mutual ecological relations, so this chemical catalogue would not be 

 representative of the cell unless it were supplemented by information on 

 the groupings, behavior, interdependence, and general operating con- 

 ditions that tie the various molecular species into a durable, and indeed 

 "viable," community. 



Biochemistry is producing an ever-growing wealth of evidence for 

 the complexity of molecular interdependence in living systems. Even 

 very elementary physiological processes, as for instance the energy 

 transfer in respiration or photosynthesis, involve a great number of 

 coordinated steps, each of which requires the presence of a definite set 

 of properly dosed reactants, mediators (enzymes), and physical con- 

 ditions (/jH, temperature, pressure). Undisturbed operation of these 

 energy-yielding systems, in turn, is a prerequisite for the synthesis, 

 among others, of the complex protein molecules, some of which, as 

 enzymes, are again fed back as indispensable links into the energy-pro- 

 ducing systems. Thus neither system can maintain itself without the 

 other. Protein synthesis itself proceeds in series of steps, each of which 



