186 ACQUIRED CHARACTERS SEC. 



the organism and the outer world, the distinction must be 

 essentially due to external influences to which is likewise 

 due the beginning of all histological evolution (differentiation) 

 in the body, to which I shall again refer. 



Ziegler believes that the causes of the origin of germ 

 variations which lead to inheritable deformities and diseases 

 may be of three kinds (1) Union of sexual nuclei which 

 are not adapted for copulation ; (2) disturbance of the copu- 

 latory process itself; (3) injurious influences which affect the 

 sexual nuclei or the fertilised ovum at a time when separation 

 of the sexual cells from the body cells has not yet occurred. 

 " If the embryo is injuriously affected at a later period, either 

 a malformation or constitutional anomaly arises, which is not 

 inherited, or only the sexual cells are injured, in which case 

 the body -cells develop normally, and a disturbance shows 

 itself only in the development of the next generation." 



The union of sexual nuclei not adapted for copulation is, 

 according to Ziegler, the most frequent and most important 

 cause of hereditary local malformations as well as of heredi- 

 tary morbid tendencies, or of a defect in any system of the 

 whole organism. "When in a family whose members show 

 no special talents a genius suddenly appears? the natural 

 explanation is, that sexual nuclei unusually well-fitted for 

 union have united in copulation, and that in consequence 

 the nervous system has reached an unusually perfect organ- 

 isation." In like manner arise tendencies to mental disease, 

 etc. If the nuclei are altogether unadapted to one another, 

 sterility results, as in the sexual nuclei of distinct species. 



Thus what Ziegler is here discussing is the self-evident 

 effect of crossing, the importance of which I fully acknowledge, 

 and on which I have already expressed similar views ; but it 

 still requires to be explained in what way the sexual cells 

 originally acquired the peculiarity of being adapted to the 

 production of a genius. 



