340 UNITY AND DIVERSITY IN BIOCHEMISTRY 



Another example connected with the general idea of phylogeny is one 

 cited by Comfort, who pointed out that the depot uroporphyrin of the shelf 

 occurs chiefly in the less specialized Archaeogastropoda. And it is possible 

 to quote many other examples, 



III. EVOLUTION OF BIOCHEMICAL SYSTEMS 



In the preceding, we have considered the constituents of an organism, 

 i.e. from the point of view of the macromolecules and organic molecules 

 contained in it. But these compounds are evidently derived from the opera- 

 tion of biosynthetic systems, and thus, a heteromorphic evolution, such as 

 the replacement of a haemoglobin by a chlorocruorin, occurs in the bio- 

 synthetic mechanism which produces that constituent. Hence this change 

 is in an enzyme system, i.e. in a system of macromolecules the nature of 

 which is controlled in every case by a gene, itself being on occasion the 

 object of a heteromorphic evolution and of a reduction in the isology of its 

 nucleic acids with those of its forbears. If we grant that the photosynthetic 

 pathway (see Part Six) is a metabolic variant of the hexosemonophosphate 

 shunt, then photosynthesis regarded as a reduction of COg will be 

 carried out by a system more specialized than the hexosemonophosphate 

 shunt. On the other hand, if it is true that at the beginning the biosphere 

 was lacking in COg, photosynthesis could only appear after the liberation 

 of this substance from volcanoes and primitive forms of metabolism. 

 If the presence of oxygen in the terrestrial atmosphere has depended 

 on photosynthesis then that part of the overall metabolic schemes which 

 concerns respiration is biochemically more specialized than that con- 

 taining glycolysis and the hexosemonophosphate shunt. But all this 

 lies in the domain of prehistoric biochemistry and consequently is 

 of a highly speculative nature. In the biosphere at the present time we 

 have before us what survives of many diversifications of the general plan 

 of cellular biochemistry described in Part III, and evidently what remains 

 has been preceded by more primitive systems which today have dis- 

 ppeared. 



(a) Specialization by Quantitative or Topographic Modifications 



Extracellular digestion compared to intracellular digestion implies that 

 specialization has occurred in the sense of a relatively great biosynthesis of 

 enzymes secreted into the lumen of the digestive tube and constantly 

 renewed. Intracellular digestion is the primitive form. It is the only form 

 of digestion in the Spongiae. 



As Yonge has emphasized, an example that demonstrates very well the 

 relation between the system of intracellular digestion and that of extra- 

 cellular digestion is the Molluscs : among them we find all stages between 

 an almost complete intracellular digestion and a totally extracellular 



