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38 THE TREND OF THE RACE 



shoulders of the Mendelian to those of his opponent. But if it is 

 granted that a characteristic is transmitted according to Mendel's 

 law it remains to be determined whether it presents a simple 

 typical illustration of such transmission or follows a more complex 

 type of Mendelian inheritance. Where several factors are in- 

 volved, inheritance, though Mendelian, may present the appear- 

 ance of the old-fashioned blending type, and should be dealt with 

 in practice as though it were truly blending. 



Let us suppose for instance that feeble-mindedness depends 

 not upon the loss of a single factor in the germ plasm, as com- 

 monly assumed, but upon the presence of many such factors 

 belonging to different allelomorphic pairs. The matings of two 

 feeble-minded persons, thus bringing together two germ plasms 

 generally tainted with defectiveness, would be expected to produce 

 nothing but feeble-minded offspring. The matings of a normal 

 with a feeble-minded person migh tbe expected to produce variable 

 results. Various factors affecting mentality in the normal individ- 

 ual would doubtless tend to give rise to various degress of mental 

 development. There would doubtless be also a considerable 

 variation in the gametes contributed by the feeble-minded person. 

 Some of the combinations of germ cells might be expected to 

 produce a much better mental inheritance than others. Add to 

 the congenital differences thus arising, other changes due to 

 intra-uterine influence, circumstances affecting early childhood, 

 and various other environmental factors, and we would get a 

 varied group whose individual members would be classed as 

 feeble-minded or normal, in proportions varying according to the 

 standard of the person making the classification and the correct- 

 ness of his judgment of the persons passed upon. Naturally the 

 categories found could be interpreted as resulting either from the 

 mating DRXRR or, in case all the offspring were considered 

 normal, from DDXRR, the normal parent being designated after 

 the usual fashion as DD or DR according to whatever assumption 

 is neccssar}'- to bring the facts into accord with the theory. It is 

 practically impossible to determine that a person is a DR unless 

 one of his immediate parents is an RR. The presence of RR's in 



