CORRELATED RESPONSE 



to be advantageous to its possessor except in so far as it can change 

 too. Since the genetic system must ahvays be under its own control, 

 an outbreeding system will be able to change in the direction of 

 inbreeding by virtue of the flexibility which is its special property. 

 The more rigid inbreeding system will be less likely to show change 

 towards outbreeding. Evidently inbreeding systems must generally 

 be in evolutionary dead ends, doomed by inflexibility to extinction 

 when a crisis arises, but always being thrown off by the continuous 

 stream of outbreeding systems. We have already seen that the 

 evidence from stratification of breeding systems fully accords with 

 this expectation. 



This evidence of stratification in the breeding systems of, for 

 example, wheat and Pediculopsis, can leave no doubt that changes of 

 system do occur. Nor can the comparison of the breeding systems 

 of relatives such as wheat and rye, or the Galeopsis species. Yet there 

 are obstacles to change inherent in the systems themselves : obstacles 

 which must slow the change-over even where they do not prevent 

 it. With the inbreeding type the obstacle, a fatal one, is, as we have 

 seen, its own inherent rigidity. With the outbreeding system it is 

 the less serious one of lack of balance of the more homozygous 

 genotypes which increased inbreeding produces in greater numbers. 

 Each system is internally consistent: change introduces a measure 

 of discordance. They show what we must describe sls genetic inertia. 



Correlated Response : Capital and Subordinate Characters 



Genetic inertia is due to integration, the building up of an 

 adaptive system providing the very properties which have been 

 responsible for its past success. This inertia of adaptation can also 

 be seen at work in other ways than in the retardation of change in 

 the breeding system. In his experiment on the scutellar bristles of 

 Drosophila, Sismanidis records that of seven selection lines with 

 brother-sister mating, three died out — through infertility. This may, 

 as we saw earher, have been the simple effect of inbreeding. But in 

 other such experiments with flies, and also with fishes, a similar loss 

 of fertility has regularly been found to accompany the change of a 

 somatic character under selection. It has done so even where out- 

 breeding was kept at a maximum. Furthermore, in an experiment 



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