INHERITAl^CE' OF DISEASE. l9 



through marriage of deterior^tirrg offspring or perpeMst^' 

 ing disease. Thiis it is said that intermarriage of fain- 

 ilies through the second generation should be avoidedj 

 and we are told that a degenerate offspring necessarily 

 ensues. Experience says that this is not absolutely true, 

 although it is often justified by facts. No one doubts] 

 however, that unions of affinity are undesirable, and 

 why? The micro-organisms which produce disease^ as 

 I have already shown, need congenial surroundings in 

 which to propagate and to flourish. The organism of 

 individuals presents some differences, and one person 

 may be a better medium for the growth of a particular 

 parasite than is another; This peculiarity is hereditary; 

 just as facial expressions are hereditary. Thus it i§ 

 tolerably certain that two persons closely related, as 

 cousins for instance, possess to some extent the same 

 favorable conditions for the development of a particular 

 disease germ. If they marry, these conditions are inten- 

 sified in their offspring. Two persons may have a ten- 

 dency to tuberculosis or consumption which is not in 

 itself sufficiently strong for development, but when the 

 combined tendency is found in a child it overcomes all 

 other opposing influences, and disease and death follow.^ 

 Two persons not related may also have predisposition to 

 disease, but in different forms. In that case the one 

 might counteract the other, and so a negative result 

 would be brought about. But if they did not counteract 

 each other, both would descend to the child, each in a 

 mitigated form — that is, neither being stronger than it 

 existed in the parent — but at the same time the off- 

 spring would inherit the weakness of both father and 

 mother, and thus its susceptibility to disease would be 

 increased; ■"■'-■ - - - : / 



Nurserymen long ago learned through "experiments 

 that pear trees budded on quince stalks dwarf ed them by 

 causing an unhealthy growth, and they also learned that 

 scions cut from dwarfed pear trees and grafted on pear 

 seedlings seldom produced healthy trees. But the graft, 

 ing of scions from healthy pear trees upon quick-growing 



