FITNESS AND FLEXIBILITY 



very few occasions, but the favouring of the mean type is in agree- 

 ment with expectation. If one extreme were favoured consistently, 

 the mean of the population would move in that direction, in so 

 far, of course, as the variation was heritable. And it would continue 

 to move as rapidly as the available variability permitted until it 

 approximated to the optimum phenotype, at least sufficiently well 

 for the environment not to favour departure preponderantly in one 

 particular direction. In the third case, where both extremes are 

 favoured at the expense of the mean, a new state of affairs arises, one 

 we shall consider in the next chapter. 



A character such as fertihty might be regarded as being in a 

 different situation, for one might expect greater fertihty to be 

 favoured almost without limit. It must be remembered, however, 

 that fertility is itself the expression of a number of sub-characters, 

 and these must be balanced against one another. To take an example, 

 man has a smaller number of offspring at a birth, or in a lifetime, 

 than the pig. But we could hardly regard an increase to the pig's 

 litter size as likely to increase the expectation of posterity in man, 

 because the success of each child requires an expenditure of parental 

 care and training which would thereby be rendered impossible. Too 

 many offspring would be as bad, though in a different way, as too 

 few. 



The adverse effect of too large a litter can be seen even in the pig 

 itself. The mortality between birth and the age of three weeks 

 becomes so great in litters of 14 and more that the number of pigs 

 surviving to this age is somewhat lower in the bigger litters than 

 it is in those of 14 and 15. At six weeks the disadvantage of too 

 large a litter is still more striking and the size of litter which gives 

 the maximum average number of survivors is even lower than at 

 three weeks (Fig. 72). In the same way, other tilings being equal, 

 the excessive production of eggs or seed by any animal or plant 

 would mean a crippling reduction in the food supply with which 

 each was endowed. Thus with fertility, too, the principle ot the 

 optimum must apply. 



What will be the effect of this principle that the average is 

 favoured at the expense of the extremes? With the inbreeding 

 system, where the population consists entirely or very largely of 

 homozygotes, the effect o{ selection in favour of the intermediate 



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