THE INHERITANCE OF DISEASE 117 



the mother directly to her infant in utero, but such process, 

 as we have already seen, is not to be confused with inherit- 

 ance proper. On the other hand, much evidence has been 

 adduced to show that natural selection has been at work in 

 evolving in certain regions immune populations, as, e.g., the 

 negroes, who are relatively immune from yellow fever and 

 ague. Infectious diseases introduced for the first time 

 among savage tribes assume their most fatal form, which 

 is evidently due to the fact that no natural immunity of any 

 degree against the introduced disease has been evolved as 

 yet in such tribes. Similar arguments hold, according to 

 some authors, with regard to alcoholism. Here, too, if a 

 certain increase of sobriety among some races has taken 

 place, it is, as Archdall Reid put it, "in proportion to the 

 length and severity of their past experience of the poison." 

 The individuals with the greatest craving for alcohol 

 succumb quickest to the ravages of alcoholism, leaving a 

 population less inclined for strong drink, and becoming 

 gradually steadier and soberer in consequence. 



III.— A THEORETICAL INTERPRETATION OF DISEASE. 



The inheritance of disease can be made clear, as Professor 

 H. E. Ziegler has shown, by means of the theory of 

 Determinants. If we imagine the tendency to disease 

 represented in the germ-plasm by tainted chromosomes, 

 then the taint will be transmitted in accordance with the 

 process of the reducing division and fertilization of tlie 

 germ-cells. As the reducing division always removes half 

 the number of the chromosomes, the germ-cells for the 

 next generation will vary as to the number of tainted 

 chromosomes they contain. The union with a second 

 germ-cell, also containing a lesser or greater number of 

 tainted chromosomes, will finally determine the number of 

 diseased chromosomes present in the fertilized ovum. A 

 small number of such chromosomes is less likely to express 

 itself as actual disease than a large number, and so on. 



