CONCLUSIONS 151 



these have not been exhaustively analysed. From our limited 

 experience it is clear that the biochemical systems within proto- 

 plasm are not uniform, i.e. there is no established biochemical 

 unity. Thus we are aware that there are systems other than the 

 Embden-Meyerhof and the tricarboxylic cycles for the systematic 

 degradation of carbohydrates; a total of six alternative methods 

 being currently available. High-energy compounds other than 

 those of phosphorus have been described; the number of vital 

 amino-acids has gone up from twenty to over seventy; all these 

 facts indicate that the biochemical systems may be very variable. 

 The morphological systems in protoplasm, too, show consider- 

 able variation. It is possible that some aspects of cell structure 

 such as the mitochondria and the microsomes might have arisen 

 independently on several distinct occasions. It is also probable 

 that two or more independent systems have evolved for the 

 separation of chromosomes during cell division. 



It is a convenient assumption that life arose only once and that 

 all present-day living things are derived from this unique experi- 

 ence, but because a theory is convenient or simple it does not 

 mean that it is necessarily correct. If the simplest theory was 

 always correct we should still be with the four basic elements — 

 earth, air, fire and water! The simplest explanation is not always 

 the right one even in biology. 



(3) The third assumption was that Viruses, Bacteria, Protozoa 

 and the higher animals were all interrelated. It seems from the 

 available evidence that Viruses and Bacteria are complex groups 

 both of which contain a wide range of morphological and physio- 

 logical forms. Both groups could have been formed from diverse 

 sources so that the Viruses and Bacteria would then be an 

 assembly of forms that contain both primitive and secondarily 

 simplified units. They would each correspond to a Grade rather 

 than a Subkingdom or Phylum. We have as yet no definite 

 evidence about the way in which the Viruses, Bacteria or Protozoa 

 are interrelated. 



(4) The fourth assumption was that the Protozoa gave rise to 

 the Metazoa. This is an interesting assumption and various 

 schemes have been proposed to show just how the change could 

 have taken place. On the other hand equally interesting schemes 

 have been suggested to show the way in which the Metaphyta 



