122 SOREN HANSEN 
and the other from manio-depressive psychosis or from Paranoia, which 
to mention a famous example, was the case with the two Bavarian kings, 
Ludvig II and Otto I (STROHMAYER, 1910). If we examine now how 
such an alternating inheritance can be explained, on the assumption, 
that both these diseases depend on a common factor and an accessory 
special factor, it will be seen that the relation between them is just the 
same as the relation between Dementia præcox and Retinitis pigmentosa, 
only with this difference that the special factor, which conditions the 
manio-depressive psychosis, must be considered to be recessive, like 
that which conditions Dementia præcox. Denoting the common, do- 
minant factor as A, the specific recessive factor of Dementia præcox 
as B and the specific and likewise recessive factor of manio-depressive 
psychosis as C, the genetic formula for a definite case of Dementia præ- 
cox may be taken as, for exemple, AaBBCc and for a definite case of 
manio-depressive psychosis, for example, as AaBbCC. Two sound 
parents with latent tendencies can thus have children with both these 
diseases in the combination: 
AaBbCc X AaBbCc = AaBBCc + AaBbCC +... 
Parents, in the case where the one is sound and the other has 
Dementia præcox, may have children with not only this disease but 
also with manio-depressive psychosis and conversely, where one is 
sound and the other has manio-depressive psychosis such parents may 
have children with this disease or Dementia præcox, and lastly, where 
both parents have either Dementia præcox or manio-depressive psy- 
chosis their children may have the opposite disease, but since the con- 
nection in this case will always be homozygotic with respect to the 
one of them, the result will be a mixed form. Such would also occur 
in the other combinations though less easily, and the important thing 
is, that every case of direct or indirect, collateral or alternating inheri- 
tance can be explained without difficulty on the assumption, that all 
these diseases are polyhybrid and united by a common factor. The 
assumption of this common factor is necessary, however, for the ex- 
planation not only of the genetic connection between the diseases, but 
especially also of the manner in which they occur, sometimes un- 
doubtedly hereditary in the family, sometimes apparently quite isolated. 
Dementia præcox and manio-depressive psychosis form together 
one main group of mental diseases, which from the principal cause 
may be regarded and designated as biogenetic in contrast especially 
to Dementia paralytica, which is of syphilitic origin, and to a few other 
mental diseases of less importance. To this principal group belong, 
