HEREDITY 



Abraham Lincoln in otherwise undistinguished families. 

 The proportion of eminent persons in such families will 

 be low because of great differences in quality among the 

 parental germ cells. On a percentage basis, selected high- 

 grade families will often produce ten, twenty, or even fifty 

 times as many notables as mediocre families. Yet the abso- 

 lute number of geniuses appearing in families of the latter 

 type will always be high because mediocrity is plentiful." 



In passing, it may be well to note that a number of papers 

 have recently appeared in which the writers have failed to 

 grasp the genetic concept of mentality. If it be true, these 

 authors say, that variations in intellectual capacity can be 

 interpreted by assuming that numerous genes combined 

 into different patterns are involved, how is it that the 

 contrast between a normal-minded person and an imbecile 

 is to be represented by a mutation in a single gene? The 

 answer is quite simple. A hundred genes, let us say, may 

 function in producing the normal mind, and a good many 

 of them must have varied into plus and minus types in 

 order to account for the diverse grades of mentality actually 

 observed. But since the evidence indicates that unions of 

 feeble-minded persons produce only feeble-minded persons, 

 one must conclude that only a single gene has become truly 

 defective, though ninety-nine other genes are left to 

 contribute to various grades of feeble-mindedness. A second 

 defective gene may yet be discovered. Proof of its existence 

 would be found in a union between two defectives which 

 yielded only normal progeny, provided genetic tests of the 

 offspring conformed with the results to be expected for in- 

 dividuals carrying two different genes for defectiveness. No 

 such evidence is available at present. 



Briefly, then, the genetic philosophy is this: Heredity 

 allots to each individual certain mental and physical 

 potentialities, governed in this allotment by the rigid laws 

 controlling gene distribution; whether these potentialities 



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