254 HEREDITY AND DISEASE 



they throw some useful side-lights on the general problems of 

 heredity. 



§ 2. Misunderstandings in regard to the " Inheritance " 



of Disease 



As with the transmissibility of acquired characters, so with 

 the transmissibility of the ills our flesh is heir to, we have to 

 face a number of current misunderstandings, which in many 

 cases obscure the real facts. The long series of transmissible 

 diseased conditions which Prosper Lucas, for instance, gave 

 in 1847, wm " not P ass muster to-day. It includes many cases 

 which are outside the rubric of inheritance altogether. A more 

 critical study, particularly of recent years, has led physicians as 

 well as biologists to define a number of distinctions between 

 real and apparent inheritance. Thus, to take a simple instance, 

 it seems a confusion of thought to speak of the inheritance of 

 any microbic disease. 



1. Reappearance not equivalent to Inheritance — The 

 reappearance of a diseased condition in successive generations 

 does not prove that it has been transmitted, or even that it is 

 transmissible. The Alpine plants which Nageli brought to the 

 botanical garden at Munich were much modified in their new 

 environment, and their descendants were similarly modified. 

 The unusual characters reappeared generation after generation, 

 but experiment showed that the reappearance was not due 

 to inheritance, but was due to the re-impression of similar 

 modifications on each successive crop. So it is with many 

 diseased states which reappear generation after generation, 

 not because they have been transmitted, but because of the 

 persistence of the unhealthy stimuli in function or in environ- 

 ment which originally evoked them. Collier's lung is a modi- 

 ficational result ; it reappears in generations of colliers, but 

 there is no warrant for regarding it as heritable. 



