IMPROVING THE HUMAN PLANT 



But, on the other hand, if the individual nor- 

 mal offspring of the first filial generation were to 

 mate with other individuals having the same heri- 

 tage, the recessive trait of feeblemindedness would 

 reappear in a certain proportion of their offspring. 



Obviously cousin marriages give opportunity 

 for the bringing together of such recessive traits, 

 and hence may cause the reappearance in the off- 

 spring of undesirable or abnormal characters that 

 might otherwise be suppressed. 



In the celebrated series of experiments made 

 by Professor Biffin at Cambridge, England, it was 

 shown that susceptibility of wheat to the fungous 

 disease known as rust is transmitted as a factor 

 dominant to immunity. 



Similarly it has been observed that immunity 

 to attacks of the aphis shown by the roots of the 

 Northern Spy apple is a recessive trait and hence 

 that the seedlings from the Northern Spy may be 

 susceptible. 



These illustrations, among others, show that 

 susceptibility and immunity to disease may con- 

 stitute a Mendelian pair of factors that are trans- 

 mitted in a definite way. There is a growing body 

 of evidence to show that the same thing is true 

 with the human subject in case of susceptibility to 

 certain microbic diseases. 



But it fortunately happens that in some cases, 



[243] 



