VIRUSES, PROVIRUSFS AND THK CONFLICT OF SYSTEMS 



come across evidence of this integration on a larger scale, at a higher 

 level, when we come to the growth of genes in evolution. 



The cytoplasmic system depends for its propagation largely (and 

 originally) on ribose nucleic acid, which, not having the indefinite 

 power of polymerization of its nuclear congener, cannot organize 

 a differentiated fibrous structure such as a chromosome. The 

 cytoplasmic units arc, therefore, liniited in organization and size, 

 corresponding in range to genes rather than to chromosomes. They 

 depend, moreover, on chemical equilibrium, co-operation and com- 

 petition, for the adjustment of their proportions and distribution. 

 The larger and scarcer ones, or the ones associated with larger and 

 scarcer corpuscles like plastids, can therefore be accidentally sorted 

 out. Again, the larger ones may be derived from smaller ones; and 

 both, in the course of development, may be derived as cell-products 

 from the nucleus; and both may in the end be lost. In these changes 

 the cytoplasmic system is subject to the nucleus. It is also, to a greater 

 extent than the nucleus, exposed to the environment and, of course, 

 to the effects of differentiation. Lastly, for their permanent trans- 

 mission, cytoplasmic elements have another channel open to them 

 than heredity: they can spread by infection. By this piracy they 

 separate themselves, in adaptation and evolution, from the organism 

 of which they form a part. And the same separation occurs when 

 by mutation they come to nmltiply in such a way as to injure or 

 kill their own mother-body. 



Comparing the two systems, we sec why the cytoplasmic system 

 has decayed as a meclianism of heredity step by step as development 

 and differentiation grew stronger. Already in the higher plants, in 

 the rogue, there is a conflict between the requirements of heredity 

 and differentiation. And in the higher animals heredity has been 

 reduced to complete, or almost complete, nuclear control. In this 

 process the efficiency of the fibrous organization of chromosomes 

 in the nucleus as a basis of heredity and of recombination has given 

 it a long-term advantage over the cytoplasm. Only in the bacteria 

 do we perhaps see an organization of life balanced between the 

 cytoplasmic and the nuclear levels. 



Within the individual there are thus systems of different kinds at 

 work in determining heredity, development and infection. In their 

 foundations, as self-propagating proteins in the cytoplasm, they are 



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