500 HUMAN BIOLOGY 



and especially in America, tuberculosis and the minor 

 infections referred to have a greatly diminished prevalence. 

 It is now to be accepted that practically all of the aforetime 

 ability to segregate a type of people having the scrofulous 

 diathesis (if such there are) was dependent on the continued 

 manifestation of the infections to which they are 

 susceptible. 



It is of interest and significance that an experimental 

 approach to the question with suitable material develops a 

 picture which fits so well with the conception of a scrofulous 

 diathesis as it stood at about the beginning of the present 

 century. There is observed in the guinea-pig experiments 

 already outhned an inherited group of reactive quahties 

 that are related to susceptibility to tuberculosis, and also 

 find expression in the character of the tissue changes in 

 tuberculosis and in some simple inflammatory reactions. 

 Respecting the hmitations imposed by species differences 

 this would seem to be as close as it could be hoped to come to 

 an experimental definition of the scrofulous diathesis. 



A generation ago the general conception of the funda- 

 mental nature of inheritance was that it was a blending or 

 fusing of the parental characteristics, stronger characters 

 being diluted by weaker. The cases which such a blend did 

 not explain were regarded as unaccountable exceptions. 

 Then the work of Mendel was revived and it was seen that 

 when inherited quahties were sufficiently analyzed into 

 their component parts the blended was rather the exceptional 

 occurrence. But instances of blending inheritance could not 

 be gotten over or disregarded and it seemed to some students 

 that there must be two principal forms of inheritance. These 

 views have been quite completely harmonized by further 

 study. In the obvious Mendehan case a particular character, 

 which to famihar scrutiny is simple and definite, is controlled 

 by the presence or absence of a single inheritable unit known 

 as the gene. Color in animals, eye color in man, tallness or 

 dwarfness in the garden pea are such characters and their 

 study clearly defined the Mendelian principle in inheritance. 

 Skin color in man if albinism is contrasted with the presence 

 of any pigment is similarly controlled. 



