68 MARRIAGE AND DISEASE. 



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I. By DIRECT HEREDITY is meant transmission direct 

 from parent to child, that is, where there is least inter- 

 ference with heredity pure and simple, and the child is 

 a compound of its parents. Prepotency may, and in 

 fact generally does,, come into play here, but there is 

 a marked absence of variation and reversion. 



This direct heredity is generally split up into two 

 sub- divisions, thus : 



1. Where the child resembles each of the parents 

 equally in its moral, mental, and physical characters 

 in fact, where the child is an exact mean of its parents. 

 But, as we have already seen, this result, which would 

 be a realisation of the ideal law, must be of extreme 

 rarity, or, if we wish to be scientifically correct, an 

 utter impossibility, for to ensure an exact mean in the 

 child would entail a perfectly equal blending of the 

 parental characters, which prepotency can seldom, if 

 ever, allow, together with a constant environment 

 which can never be obtained. We do occasionally meet 

 with a case where the child appears to be as nearly as 

 possible a mean of its parents, but even in such cases 

 it is never difficult to discover that the mean is far 

 from perfect or exact. For this reason this form of 

 direct heredity needs no consideration at our hands. 



2. Where the child, being a compound of its parents, 

 resembles one parent more stronr/ly than the other. In 

 most cases of direct heredity the child resembles some 

 one of the parents much more strongly than the other, 

 and this is only what we would expect from what we 

 have already learnt of prepotency. But although the 



