278 THE CAUSATION OF DISEASE. 



disease. When due to pathogenic germs, a disease may, it is 

 true, evolve by natural selection or otherwise, since here the 

 specific mal-E is a living organism, which in mode of evolu- 

 tion differs in no wise from other organisms, but in such cases 

 the evolution belongs to the E and not to the S. When, 

 however, we speak of the evolution of disease, we are not 

 referring to the evolution of a pathogenic organic E, but of a 

 structural state on the part of the suffering organism — a 

 change wrought in the S, it may be during several successsive 

 generations. 



Repeated allusion has been made to the influence of a 

 mal-E operating during several successive generations. All the 

 necessarily-fatal-E's which do not kill in the first generation 

 operate in this way. There is a gradual deterioration in struc- 

 ture which eventually leads to family extinction, and in such 

 cases there may perhaps be said to be an evolution of disease. 

 The S during successive generations is gradually modified in 

 such a way that presently S + a particular E, which perhaps 

 would have no ill-effect upon a healthy individual, induces a 

 fatal disease. 



In this way many disease-tendencies — e.g., to insanity and 

 gout — are slowly evolved. 



Intemperance is so well known to be a fruitful cause of 

 insanity that we might, in certain cases of this disease at all 

 events, have little difficulty in tracing its evolution during 

 successive generations and in determining its proper cause, 

 and the same is true of gout and other disorders. But how 

 many are the subtle causes of disease which quite elude our 

 observation ! Generation after generation of men exposed to an 

 ever-operating but unknown E may in this way be gradually 

 and imperceptibly changed in the direction of disease. Such 

 changes are of course structural, and constitute natural varia- 

 tions ; they are wrought in the finer textures of the individual, 

 and for long may lead to no obvious change, but finally, after 

 several generations, the specific E calls out the actual disease, 

 which is then recognizable as a true pathological variation. 



Now it is interesting to note that this is exactly what 

 happens in the case of ordinary physiological variations. A 

 gardener has often to alter the E of a plant for many 



