234 GENETICS 



bination of factors which is the cause of the parental 

 defect on either side of the pedigree does not happen to 

 recombine after segregation to form the new individ- 

 ual. Deafness will be produced in the offspring only 

 when matings occur in which the proper factors are 

 combined. Such an undesirable result is much more 

 likely to happen if both parents come from the 

 same, or related, hereditary strains than if they are 

 derived from families in no way connected by blood. 



Herein lies the biological objection to cousin 

 marriage which tends to bring together, and thus 

 to perpetuate, like defects. Outcrossing, on the 

 contrary, through the law of dominance, tends to 

 conceal defects and to prevent their expression. 



Many other cases of human defects, such as im- 

 becility or insanity, are extremely difficult of analysis 

 from the standpoint of heredity because, in the first 

 place, the defective conditions descriptively included 

 under these vague terms are made up of a multitude 

 of diverse conditions each of which must have a 

 different array of determiners and, in the second 

 place, because any one definite sort of insanity or 

 imbecility may be conditioned by a variety of 

 factors. 



However, the difficulty of the problem is no 

 reason for abandoning the attempt to reach its solu- 

 tion and to learn, if possible, "whence come our 

 300,000 insane and feeble-minded, our 160,000 blind 

 or deaf, the 2,000,000 that are annually cared for by 

 our hospitals and Homes, our 80,000 prisoners and 

 the thousands of criminals that are not in prison, 



