510 HUMAN BIOLOGY 



that ill no single instance does it furnish a pertect insight 

 into the fundamental nature of even one of these inheritable 

 units. The task for the future is obviously enormous if we are 

 to gain a usable understanding of the inheritance of disease 

 on the basis of rational knowledge. We require to know for 

 the different disease conditions the precise part played by 

 the inheritance in toto; the number of unit characters in- 

 volved for each case, and their structural or functional nature. 

 It may well be, however, that the obstacles which intervene 

 between our present understanding and a much more perfect 

 and useful one are lessened by some favoring circumstances 

 which may be sketched. 



While it is considered fundamental that unit character is 

 distinct in inheritance, certain definite instances are known 

 where diverse characters are usually inherited together. This 

 is termed Hnkage. Thus in hemophiha (which is manifest 

 by failure of the blood to clot, so that those affected are 

 "bleeders,") the disease condition is finked with the factors 

 determining the sex. It is also true that a single unit character 

 is sometimes known to be concerned with a variety of 

 structures or functions although the author is unable to point 

 out an example of this nature with reference to any disease 

 condition. 



From the point of view of pathology, also, there are rather 

 clearly outfined associations between certain structural 

 pecufiarities and disease conditions, excluding cases pre- 

 viously outfined where the disease is directly dependent on a 

 particular fault of structure. There are also recognizable 

 tendencies for individuals and famifies to suft'er from or be 

 relatively immune to groups of diseases. Thus the tafi, thin, 

 flat-chested type of man is befieved to be more fiable to 

 acquire tuberculosis. People who suffer from rheumatism 

 and gout are befieved to be less fiable than the average to 

 acquire tuberculosis. Most of these relationships are, as at 

 present recognized, of the uncertain order resting on the 

 impressions of successive generations of physicians. Yet 

 recent approaches to the subject on the basis of careful 

 measurements, accurately recorded case histories and 

 adequate statistical analysis lend credence to the befief that 

 there is a real and traceable set of associations here which it 



