336 AN INTRODUCTION TO MODERN GENETICS 



another disease involving strong mental symptoms, and is probably 

 caused by a recessive. In other cases of more or less defined pathological 

 conditions, such as microcephaly, the hereditary determination is still 

 obscure; and there are still other kinds, such as Mongolian idiocy, 

 where there is evidence that environmental conditions are very impor- 

 tant (Mongolian idiots tend to be more frequent among children born 

 late in their mother's Hfe). Many cases of very low-grade mentality 

 belong to less well-defined types, and evidence as to their hereditary 

 nature is still rather vague. But it is suggestive that in an overwhelming 



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■jO o I ■ | -o ■ 

 i 6 i 6 i-ro 6 4jO ik i-ro 6 



Fig. 139. A Pedigree of the Darwin Family as an example of how intelligence 

 "runs in families." — Males squares, females circles. The males marked in black 

 attained intellectual eminence, most of them having been Fellows of the Royal 

 Society; no attempt has been made to assess the intellectual achievements of the 

 women. Charles Darwin is marked with an X; note that he married his cousin, 

 and that the inbred family was extremely successful. 



(After Holmes.) 



proportion of cases in which one of a pair of identical twins is mentally 

 defective, the other, which must have exactly the same genotype, shows 

 a similar type of defect. Moreover, matings of two low-grade defectives 

 tend to give a great majority of low-grade deficients. Probably multiple 

 factors are involved. 



When we consider the sub-normals, normals, and geniuses the un- 

 ravelling of the genetical situation becomes more important and more 

 difficult. There are several well-known pedigrees which exhibit an 

 apparent inheritance of either superior or inferior mentality. Famous 

 cases of the former are the Darwin family, of the latter the Jukeses or 

 the HiU-folk. 



But we may ask how far this inheritance is genetic or how far it is 

 due to the persistence of a favourable or unfavourable social environ- 

 ment. We require to know how much difference can be made by the 

 range of environments which actually occur in human society, and how 



