RESTRICTION AND FLEXIBILITY 



between the genes of the parental sets, genetical flexibility wiD be 

 maintained and prospective readaptation will remain a possibility. 

 Even a new inbreeding polyploid can for many generations retain 

 some flexibility, through the gradual unloading of the differences 

 between its diploid ancestors. 

 We can thus see that complete restriction of recombination is 



Regular cross-fertilization within a 

 MATING CONTINUUM. Heterogeneity 

 and heterozygosity due to floating changes 

 (1) structural and (2) genie. (1) interfering 

 with recombination of (2). 



Break-up into many INBREED- 

 ING or obligatorily self-fertilizing 

 units which are potential new 

 species since no gene exchange 

 occurs between them. 



Adaptive combination of specific 



genes with specific structural 



types now floating jointly. 



APOMIXIS by progressive 

 or sudden suppression of 

 recombination, i.e. un- 

 conditional fixation. 



Fixation of heterozygotes 

 with cumulative complex 

 differences (XY and 

 Oenothero). 



Fixation of homozygotes by 



splitting into pairs of 



species each a new mating 



continuum. 



Fig. 8i. — Scheme of genetic changes in relation to species formation. 



final and suicidal. As usual, it is the compromise solution, the partial 

 and adjustable restriction, which is ultimately the most successfid. 

 The desirable genie combinations are held together sufficiently well 

 for present needs by the abolition, or near abolition, of breeding 

 between populations and of recombination between structural types; 

 but the possibility is retained o( further adjustment either within 

 the population or within the structural type. Each population or 

 each structural type is separated from its fellows, but remains itself 

 as defining a new and reduced mating continuum. This must start 



319 



