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variability in disease are seen in the differences of individual 

 constitutions, as to their mode of manifesting the effect of 

 the same morbific cause, and their relative power to sustain 

 it, and in the metamorphoses or transformation in trans- 

 mission, to which I have recently alluded. The great prin- 

 ciple for which I am contending is the due recognition of 

 individuality in every case presented to us, remembering 

 always that physical and mental differentiation necessitates 

 the differentiation of pathological processes ; and that, co- 

 operating with the great law of heredity which tends to 

 produce like from like, is the equally potent, if subservient, 

 law of variability, which necessitates our differentiation from 

 each other physiologically, psychologically, and pathologi- 

 cally, and by means of which our individuality is developed 

 and maintained. 



Diseases of the Respiratory Organs. In proceeding to con- 

 sider the hereditary element in the diseases of the respiratory 

 organs, I shall not content myself by merely stating that 

 certain of these diseases are distinctly hereditary, but 

 taking a broader view of them all, I contend that, for 

 reasons insisted upon over and over again in the foregoing 

 pages, there is a predisposition of tissue inherited by those 

 who suffer from any of them, as, indeed, there is, and 

 must be, in all morbid processes which are not distinctly 

 acquired. As I have already stated the nature of these 

 predispositions is unknown to us, but of the following fact 

 there can be no doubt viz., that physiologically, psycho- 

 logically, and pathologically, also, the textural and func- 

 tional peculiarities of ancestors and parents are handed 

 down to their descendants by heredity in varying pro- 

 portions, modifiable by variability and the influence of 

 external circumstances. The law of heredity is inexorable, 



