328 THE CAUSATION OF DISEASE. 



ascertained if we possessed a family history dating back 

 a few generations only, and the same remark applies here. 

 Nevertheless, when inquiring into the hereditary history in 

 cases of disease in man, we are only able to go back through 

 the most recent generations ; indeed, Sir James Paget, with 

 his large experience, tells us that he has never been able to 

 trace the family history beyond five, and that very rarely. 



And here let me briefly indicate some of the apparent ex- 

 ceptions to the principle of heredity. According to our 

 principle, differences other than the sexual between brothers 

 and sisters are due to differences in the E, this E dating 

 back to the earliest period of germ and sperm life. A very 

 small initial difference in the E may ultimately lead to a very 

 wide divergence. The differences of ante-partem E account 

 for the differences among brothers and sisters at birth, but 

 in the present state of our knowledge it would be useless for 

 us to attempt to explain minutely the influence of ante-partem 

 E on the child. We can, however, trace the influence of post- 

 partem E more exactly, and I propose to touch briefly upon 

 this influence, in order to show how it tends to obscure the 

 hereditary principle. 



Occasionally we see among children of the same parents 

 great differences as regards mental faculty. One may grow 

 into a remarkably clever man or woman, while another may 

 exhibit very ordinary ability. Now, differences in mental 

 power depend upon differences in brain organization ; and 

 inasmuch as this is inexpressibly subtle and complex, we can 

 understand how a very slight influence may interfere with 

 cerebral development, and thus alter, in a very marked degree, 

 for good or for evil, the mental characteristics of the indi- 

 vidual. And the change wrought need not be visible to the- 

 naked eye, or even with the aid of the microscope. The subtle- 

 ties of thought are represented by subtleties of organization 

 quite out of the reach of observation ; so that, setting aside 

 the ante-partem E — which, however, I doubt not, plays the 

 more effective part — we can understand how, during the post- 

 partem life of a child, many influences, silent and unobserved, 

 may be modifying cerebral organization. But some of these- 



