CHAPTER IX 

 THE FUNCTION OF SEX 



Most Mendelian characters are sexual characters The parallel between 

 Mendelian and sexual characters It has been said that the inheritance of sexual 

 characters is Mendelian The truth is that the inheritance of Mendelian char- 

 acters is sexual The function of sex if (i) the Mendelian hypothesis is true, 

 (2) if the mutation hypothesis is true, (3) if evolution is founded on fluctuations and 

 inheritance is blended. 



M 



308. -|\ /fUTATIONS and Mendelian reproduction are 

 realities. If, then, the organism is a bundle of 

 adaptations, and evolution builds on fluctuating 

 variations which blend, how is their presence to be accounted for ? 

 We have considered several great problems of heredity and 

 evolution, for example the nature of the distinction between 

 classes of characters, the mode in which the individual develops, 

 the mode in which species evolve, the causation of variations, 

 and the like. Obviously the mutation hypothesis is concerned 

 with none of these except evolution. But evolution is merely 

 another name for the adaptation which results from progression 

 and retrogression ; and, as we have seen, it is not believable that 

 mutations, which occur only rarely and which, speaking practically, 

 are never adaptive, can have furnished the materials for the 

 multitudinous, the perpetually changing, the infinitely complex 

 and delicate adjustments of living beings. We are driven, then, 

 to the conclusion that they are nothing other than abnormalities, 

 extreme variations which arise only because evolution (and there- 

 fore the regulation of variations) is never perfect just that class of 

 variations which nature checks by the weeding out of the exception- 

 ally variable individuals, but which human breeders encourage and 

 preserve, that class on which surgeons expend the resources of their 

 art to eliminate in the individual but to perpetuate in the race. 



309. We have to delve deeper before we can account for the 

 Mendelian phenomena. Experimental workers assume that in 

 Mendel's laws they have acquired a master-key to all or most of 

 the problems of heredity. But, if we consider the question closely, 

 it becomes evident that Mendel's laws are concerned with nothing 



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