CONCLUSION 



of propagation, from which a hierarchy in control is inevitably 

 derived. For the mechanically stable strings of genes with their 

 potentially permanent organization, their co-ordinated reproduction, 

 their controlled mutation, and their regulated recombination by 

 crossing-over, acquire a sensitiveness to selection, and hence a 

 capacity for adaptation, such as is bound to give them control of the 

 unorganized proteins of the cytoplasm, which depend on their 

 individual chemistry for their proportions. The nucleus, no doubt, 

 has step by step acquired the property of putting together, after due 

 delay, the materials given it by the cytoplasm, to make larger protein 

 molecules. In these circumstances the cytoplasm is bound to appear 

 as the weaker partner. On the other hand, the variable and 

 controllable cytoplasm provides the necessary vehicle for a 

 differentiation which could not be directly related to the permanent 

 and rigid organ of heredity. For separating the two controlling 

 instruments, for stabilizing the government of each, for regulatmg 

 differentiation and for delaying interaction, the nuclear membrane 

 is the king pin of the whole mechanism. 



We cannot, however, pursue our investigations downwards to 

 the limit without considering the results of carrying them to the 

 higher level. It was at the higher level of populations, races, species 

 and the larger classes of animals and plants, and their changes over 

 the greater spaces of time, that Darwin was able to see principles 

 which arc more obscure within the individual with its short span 

 of life. These principles wc are now able to see with a new clarity 

 in the light of genetics. We arc able to see the interlocking of the 

 heredity^of the individual with the variation of the group. We are 

 able to see genetic systems as composed of mating habits, re- 

 productive mechanisms, hereditary materials, and in man of 

 traditional cultures as well, which are adapted to the needs of 

 groups large or small, as the case may be. These adaptations arise 

 from selection which acts, most obviously on individuals, but in its 

 long-range effects on larger and longer-lived units, and in its short- 

 range effects on smaller and shorter-lived units such as single cells. 



The three levels of integration of cells, individuals and populations 

 correspond as we thus see not only to differences in size of units 

 but also of their duration in time. And heredity, variation, and 

 selection act at all three levels in respect of both size and duration, 



370 



