THE UNIQUENESS OF THE INDIVIDUAL 



importance was not made until 1928. In that year Fred. 

 Griffith, a bacteriologist employed by the Ministry of Health, 

 wrote a carefully reasoned paper in which he recorded the 

 following surprising fact: that if a mouse were inoculated with 

 a mixture of living pneumococci of one type and dead pneumo- 

 cocci of a second and quite different type, there would arise in 

 the mouse living pneumococci of the second type. Looking 

 back, we can now see that he had accomplished something far 

 more important and richer in possibilities than the transmuta- 

 tion of elements; in principle at least, he had accomplished the 

 transmutation of a biological species. Others pursued his 

 discovery. First, it was found that the transformation could 

 happen in a test-tube, not necessarily in a mouse; then, that 

 pneumococci of one type could be transformed into pneumo- 

 cocci of a second type by an extract from the latter, i.e. not 

 necessarily by whole dead cells. O. T. Avery and his colleagues 

 have since sho^\Ti that the transforming principle is a desoxy- 

 ribonucleic acid, a nucleic acid of the type which, in higher 

 organisms, is confined exclusively to the nuclei of cells. Up- 

 wards of twenty such examples of genetic transformation are 

 now known, each one mediated by a particular nucleic acid. 

 The nucleic acid behaves as if it were a ''gene'': it enters the cell 

 by a process analogous to infection, takes its place among the 

 other determinants of heredity, and like the other determinants 

 is continually reproduced anew. Viruses and bacteriophages 

 are also nucleoproteins and they too act essentially by bringing 

 about a cellular transformation. It is probable that the trans- 

 formations brought about by viruses are essentially similar to 

 those produced by a solution of nucleic acids; indeed, when a 

 bacteriophage enters a bacterial cell, it is believed to leave its 

 protein moiety outside, as if the protein were the mortal part 

 of its constitution; only the nucleic acid goes inside. 



For these weighty reasons, it is now widely believed that the 

 nucleic acid moiety of the nucleoprotein molecule is that which, 



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