7 6 THE CAUSATION OF DISEASE. 



the upper hand in some respects ; those of the male element in 

 others : the final result being a structural product which is a cer- 

 tain mean between the two. I am contending that this particu- 

 lar mean, whether it takes chiefly after the male or the female 

 parent, would not be departed from if the E were in each case 

 the same. What it is that specially determines this u pre- 

 potency " is a difficult matter to decide, but I shall return to 

 this question presently.* 



The principle of structural mean applies equally, of course, 

 to normal and abnormal structures, and has therefore an im- 

 portant bearing upon disease, for it opens up the question : 

 what will be the effect upon the offspring of the union of a 

 healthy with an unhealthy individual, or the union of two un- 

 healthy individuals, having either the same or different diseases 

 or disease tendencies ? And this leads us to the interesting 

 subject of hybridization or mongrelization of disease. 



Viewed from the pathological standpoint, these blendings fall 

 under three heads : — 



1. Physiological blendings. 



2. The blending of normal with abnormal. 



3. The blending of abnormal with abnormal. 



1 and 3 admit of subdivision into three classes. I should 

 observe, however, that such division, though logical, is perhaps 

 too mechanical to serve a practical purpose. We may, neverthe- 

 less, I think, adopt it provisionally. These subdivisions are — 



a. Blendings of the same peculiarities of the same tissue. 



b. Blending of different peculiarities of the same tissue. 



c. Blending of different peculiarities of different tissues. 

 Thus— 



la. Here the blending characters are normal, of the same 

 kind, and affect the same tissue. Thus, father and 

 mother may have great strength of limb, strong diges- 

 tion, genius for music or art. 



lb. The blending characters are normal, affect the same 

 tissues, but are of unlike kind. Father has taste for 

 poetry, mother for mathematics. Father large hands, 

 mother small hands. 



ic. The blending characters are normal, but unlike, and 

 * Vide Chapters XI. and XII. 



