CONCLUSION 



have already intruded themselves. The theory of selection is one, 

 and the theory of human society is another. We may call attention 

 to yet a third, which we may call the gene-recombination complex. 

 The mating continuum depends for its function, as much as the 

 nucleus depends for its predominance, on recombination. Discon- 

 tinuity between permanent chromosomes arises when recombination 

 is obstructed. But recombination again depends on activities at a 

 higher and a lower level. In the first place, there must be crossing 

 in the population between individuals which differ in heredity. In 

 the second place, there must be segregation and crossing-over in the 

 cells of heterozygotes produced by this crossing. The segregation 

 and crossing-over system in turn presents us with its own conflicts; 

 for crossing-over in all organisms is a condition of pairing and 

 segregation (at least in one sex). And crossing-over is restricted both 

 in its frequency and in the size of the units it recombines by the 

 extent of the differences between mating chromosomes. This is due 

 to the existence of two modes of differentiation of chromosomes, 

 the axial or structural and the lateral or genie. The greater the 

 structural differentiation, the smaller the crossing-over. The result 

 of this system is to buffer the amount of effective recombination, 

 which is a function of the product of the amount of difference and 

 the frequency of crossing-over within, and random assortment 

 between, chromosomes. 



These interactions take effect in a wider field. We have seen them 

 in some variety determining how discontinuities of different sizes 

 may float in the flow of variability or become stranded in the 

 formation of races and species, the gene being trapped by the 

 inversion and the inversion trapped by the environmental dis- 

 continuity. We have also seen them in a different way determining, 

 under the guidance of selection, the growth, and the organization of 

 genes over periods of time much greater than those responsible 

 for the origin of species. 



The examination of the growth of genes has indeed given us a 

 ghmpse of a new world of which as yet we know httle, except that 

 it is probably as complex as that of populations. We can see, in 

 progenies, in chromosomes, and in cell products, the broad outlines 

 of the distinction which separates the polygenes, the major genes 

 and the super-genes. We can also see how the polygene can be 



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