Unbalanced Growth 

 and Death 



A STUDY IN THYMINE METABOLISM 



SEYMOUR S. COHEN 



In a volume presented to Sir Frederick Gowland Hopkins in 1937, 

 N. AY. Firie offered an essay entitled "The Meaninglessness of the 

 Terms Life and Living." This effort proved to be quite convincing and 

 had the general effect of eliminating those terms from the working 

 vocabularies of a whole generation of biochemists and biologists. As 

 a biochemist working on viruses, Pirie had found it necessary to be 

 more precise in the description of these organisms than the usual defi- 

 nition of "life" and "living" made possible. 



In more recent years, Pirie has discussed various aspects of the 

 problem of biopoesis, or the origin of life, having presumably been led 

 to this subject by a consideration of the question of the evolution of 

 viruses and of their cellular hosts. The problem of the origin of living 

 systems is now moving to the stage of experimental study, and it is 

 evident that the goal of such an experimental program must be de- 

 fined if a successful conclusion is to be recognized. It is no more than 

 fair, therefore, even if amusing, that, as the need arose, Pirie should 

 have undertaken to restore "life" to our working vocabulary. In the 

 absence of a calculus of biochemistry, which could facilitate a quan- 

 titative description of the coming into being of living matter, it was 

 reasoned that the least which must be done was to define the minimal 

 major characteristics of living matter. Living substance has, therefore, 

 been defined by Pirie as a system, containing liquid and catalytically 

 active matter, which is capable of growth and reproduction. 1 



In the last few years we have observed some phenomena for which 

 a satisfactory terminology is rather difficult to find. One might adopt 

 the approach of Humpty Dumpty, "When I use a word, it means just 

 what I choose it to mean — neither more nor less." Nevertheless this 

 has seemed rather dangerous in this instance, and I have seized upon 



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