THE CAUSATION OF DISEASE. 121 



be truthfully said ? Modern civilization has wrought the nervous 

 system into a high state of instability. Hence, in the union of 

 an individual having a tendency to insanity with an average 

 individual, the combining tendencies would be more or less in 

 the same direction. Evolution in the case of each having pro- 

 ceeded quite recently upon lines more or less identical, there will 

 be no sharp divergence as regards the nervous system. I hold, 

 nevertheless, that the offspring would show a strong tendency 

 to reversion if the non-affected parent presented a perfectly 

 vigorous and stable nervous system ; and I should not consider 

 this position false, without the strongest evidence, in any sup- 

 posed instance of the contrary, of this parental perfection. The 

 sole crucial test would be the union of a highly neurotic individual 

 with a healthy savage. Let it be noted, however, I do not for 

 one moment assert that there would be no tendency in such a 

 case to the inheritance of the weakness. Brown- Sequard's 

 experiments on guinea pigs would at once disprove any such 

 assertion. 



I imagine that this tendency of ' ' crossing " to produce a 

 status quo ante plays an important part in the inheritance of 

 disease, and may help to explain how the same intensity of the 

 same disease in the one parent is in a certain number of cases 

 strongly inherited, but in others absolutely uninherited. For this 

 frequently occurs, and leads one to think that the principle of 

 heredity is very fickle. 



The following case illustrates this point : A. suffered from 

 ichthyosis inherited through his father's side. He married 

 twice. The children by his first wife were quite free from the 

 complaint. Those by his second wife — three in number — 

 were all afflicted with it. It is manifest that the integu- 

 mental system in the first wife, or some tissue or tissues govern- 

 ing it, prevented the inheritance of the defect. Therefore, 

 putting aside all theory, we have distinct evidence that certain 

 crossings are capable of diminishing the inheritance of a 

 particular disease; and it would, indeed, be a great triumph 

 if we could discover what particular order of constitution is 

 capable of weakening or obliterating, on this principle of cross- 

 ing, the inheritableness of different diseases — if we knew, for 

 instance, what body-habit would prevent rheumatism in the 



