by the interaction of heredity with the law of variability, and 

 so it must be pathologically for the reason I have elsewhere 

 stated viz., all morbid action is but a modification or per- 

 version of some natural or normal action or function ; and 

 all the physical results constituting morbid structural altera- 

 tions are mere perversions or modifications of natural or 

 normal textures, or, at most, analogous textures fabricated 

 from the same materials by like processes. Every living 

 individual, therefore, differing as he does from every other 

 physiologically and psychologically, must also differ from 

 every other in his pathological tendencies ; and I believe 

 that ignorance of this fact is the most fruitful source of false 

 views both in pathology and in practice. As too much stress 

 cannot be put upon my argument in this direction, I once 

 more briefly summarise it here. By the law of heredity, all 

 living beings tend to reproduce themselves in their offspring 

 physically, mentally, and morbifically ; but owing to the law 

 of variability which interacts with that of heredity in the case 

 of every individual, the likeness is never perfect the ideal 

 law of heredity that like produces like is never attainable. 

 We see this even in the case of twins, whose resemblance to 

 each other, however striking, is still far from perfect. 

 However much a child may resemble its parents or one 

 child another essential differences will be found externally 

 and internally, physically and mentally, and with regard to 

 their pathological tendencies. As in health so also in disease : 

 Nature preserves the type by varying the individual ; and as 

 in health so also in disease, the individuality of every living 

 man is necessitated and maintained by the law of variability. 

 The phenomena of variability in health are seen in the 

 physical and mental differentiation of every individual from 

 every other, however closely allied ; the phenomena of 



