THE THREE-LEVEL SYNTHESIS 



changed, shitted or destroyed without danger and with the possi- 

 bihty of creative adaptation, while the larger units can be broken 

 down only with disastrous effect. In other words we can see dimly 

 the delicate framew^ork of the evolution of genes on whose handling, 

 often rough handling, we depend for our experimental effects. 



We also see, in place of the abstract gene of Johaimsen, a material 

 particle; a particle, however, which is no longer the biUiard ball of 

 Morgan, but a unit whose size is functional and depends on the 

 nucleus in which it lies, the activities in which it is engaged, and 

 the linear correspondence of the partner with which the chances 

 of the mating system have given it the opportunity of crossing-over. 



On this view — and no simpler view will comprehend the evidence 

 — the gene, like the species, is a growing and diversifying and, of 

 course, unstable structure continually adapting itself under the action 

 of selection. Amongst self-propagating units the one barrier which 

 seems almost irremovable is that between those inside and outside 

 the nucleus. Outside, the absence of fibrous structure maintained 

 by chromosome nucleic acid limits the unit to something comparable 

 to the smallest genes. These may multiply, as viruses do, to form 

 large molecules, but not to permit the internal differentiation of 

 a chromosome, still less of several chromosomes. The fibrous 

 organization of chromosomes has given them an advantage over 

 the rest of the cell, which they have clearly never lost since cellular 

 differentiation was first established. Indeed, we may say that sexual 

 reproduction, with its crossing-over, is necessary to make the 

 chromosome work, and the chromosome is necessary to make the 

 cell work. 



Thus the principles of life are not subdivisible according to the 

 same rules as those for subdividing the techniques used for discover- 

 ing them. The old barriers are now breaking down, and for the first 

 time we can see biology as a whole. We can see a system working 

 subject to four primary laws (of heredity, variation, selection and 

 adaptation), at the three levels of integration, between the molecule 

 and the species, otherwise subject to different laws at different levels, 

 but no longer differentiated in regard to these laws as between 

 plants and animals, or even at the lowest level as between liercdity 

 and development. 



We began our study of genetics with tlic modest intention oi 



373 



