IV MENTAL DISEASES 191 



very different conditions in this respect are to be observed 

 — conditions which form the most important peculiarities, 

 constitute a part of their character. Many nations which 

 have become exhausted by their own civilisation, or are 

 becoming exhausted, prove this clearly enough, and there are 

 examples obvious enough and even belonging to most recent 

 times which I need not particularise. Such nations are some- 

 times conscious that they need to be reinvigorated by the 

 introduction of healthy blood from another race. Crossing is 

 here the only possible remedy remaining. But the disease 

 can only be explained by the aid of the action of external 

 influences upon the nervous system. 



It is actually inconceivable that the system of organs, 

 which not only is the instrument of the relations of the body 

 to the outer world, but which of necessity owes its origin 

 and its evolution to these relations, should not be capable of 

 undergoing hereditary morbid changes in consequence of such 

 relations when they are injurious. 



It is intelligible that the direct action of external conditions, 

 or certain states of the nervous system which are ultimately 

 ascribable to injurious external influences, lead to "fixed 

 ideas," to the tendency to self-accusation, to melancholy and 

 suicide, and that these aberrations may be hereditary seems 

 certain. But the origin of these aberrations cannot be com- 

 prehended unless they are acquired through relations to the 

 outer world. It is therefore inconceivable that in germ-cells, 

 whose ancestors were never exposed to corresponding in- 

 fluences, the conditions for the origin of these aberrations 

 should suddenly occur. For mental diseases have their seat 

 in the brain, and the earliest ancestors of man must have 

 acquired the brain itself through relations to the outer world 

 — and only these relations could produce those diseases in it. 

 It is therefore certainly intelligible (even if it be not admitted 

 that melancholia, for example, is a disease depending on 



