The genetic mechanisms described in Chap. 3 presumably evolved 

 because they preserved successful combinations of genetic material. 

 Some protoorganisms may merely have continued growth until acci- 

 dents led to their disintegration. Many may have died because 

 changing surface-volume relationships disrupted their inefficient 

 internal organization. Some may have fragmented into smaller en- 

 tities, with chance alone determining whether the offspring frag- 

 ments would have the organization to continue growth. Any mech- 

 anisms arising by chance that would tend to ensure that subsequent 

 fragmentation products retained the capacity for growth (and 

 further successful fragmentation) would automatically be perpetu- 

 ated. Thus evolved the mechanisms that led to a stabilization of the 

 marked variation which must have occurred in early division and 

 development. The origin of these mechanisms is, in a sense, the 

 basic problem in the origin of living systems, as has been discussed 

 in Chap. 1. It has been facetiously suggested that human beings are 

 merely one means that DNA has evolved for making more DNA; it 

 may also be said that DNA is merely one device used by human 

 beings to keep from having nonhuman offspring. Genetic material 

 does not replicate without other components of living systems. The 

 course of evolution has involved increasingly complex systems, in- 

 cluding the genetic one. 



GROWTH AND HOMEOSTASIS 



Presumably the earliest organisms were unicellular (or noncellular). 

 In such organisms only one or two cell divisions (and possibly one 

 fusion of cells ) produce separate functioning entities. Here the dis- 

 tinction between heredity and development or differentiation that 

 we are accustomed to draw for multicellular organisms is often 

 difficult to make. Each cellular component is a hereditary unit that 

 is replicated with greater or lesser accuracy during cell multiplica- 

 tion. 



With the development of multicellularity and increased com- 

 plexity, other problems arise. There is eventually a separation of 

 germ-line cells and somatic cells. Nuclear and nonnuclear replicable 

 components of cells that were present in unicellular organisms now 

 appear to diverge somewhat in function. Greater stabilization and 

 control are characteristic of the nuclear material, which we think of 

 as the genetic information (genotype). The nonnuclear material 

 plays a major role in development and differentiation, changmg 



57 



