THE CAUSATION OF DISEASE. 39 



What I particularly want to emphasize is the fact that all 

 variations, be they pathological or physiological, fall under the 

 one great class ot" natural variations, and it is by no means easy 

 to sharply separate the two. In the true-pathological varia- 

 tions, however^ the structural change is probably always one of 

 dissolution. 



It is clear, then, that the two great powers which deter- 

 mine structure are Heredity and Environment. We may thus 

 frame two important propositions, and these together will serve 

 as a text for this portion of our subject. 



1 . If " heredity " were unhampered, there would be com- 

 plete identity among the male offspring born of the same 

 parents on the one hand, and among the female on the other, 

 each structural type being a certain mean between the 

 parental structures. 



2. This identity is destroyed by a diversity of E. 



Each of these two factors, Heredity and Environment, must 

 now be considered in greater detail. The latter thus comes in. 

 for a second consideration — this time as a power capable of 

 influencing structure. I shall treat of them separately, so far 

 as possible, but it will bo quite impossible to keep them 

 rigidly apart. 



