THE BREAKDOWN OF CONTINUITY 



of the mating continuum and must be met by different genetical 

 adjustments. The mating continuum of any group in a differentiated 

 environment therefore has an iiilierent instabihty. How is this 

 instability resolved ? 



The instability arises from the exchange of genes between 

 individuals and groups requiring different local adaptations. As fast 

 as the local conditions select and separate more favourable combina- 

 tions of genes, these will be broken down and brought back towards 



Fig. 76. — The distribution of a recessive mutant determining "simplex" teeth in 

 the field mouse Microtus arvalis, according to the percentage of homozygous 

 individuals (after Zimmermann, 1937). 



the common level by the intrusion, from some other part of the 

 continuum, of other combinations which have been selected under 

 different conditions and hence adjusted in a different way. As fast 

 as advantageous combinations of genes arise by segregation and 

 recombination, they will break down by the same processes. The 

 flow of variability, which arises from segregation and recombination, 

 is necessary for the production of new and superior combinations 

 of genes. Once, however, these combinations have been achieved, 

 the means of their origin immediately becomes not merely 

 urmecessary; it becomes actively harmful. The value of recom- 

 bination lies, in fact, in bringing together combinations of 



304 



