L 
120 SOREN HANSEN 
of a different localisation of the same diseased process. To some 
extent it is unnecessary to assume, that the deafness genetically con- 
nected with Retinitis pigmentosa depends on a specific basal factor, 
but it may be maintained that it is a special form which just through 
its genetic relation differs from other forms of inherited deafness. 
What is known so far regarding the inheritance of deafness or deaf- 
mutism is not based, and owing to the great diagnostic difficulties 
cannot be based, on sufficiently detailed knowledge of the causes in the 
individual cases. Many of the apparently inherited cases of inborn 
deafness rest obviously on diseased changes of such a nature, that they 
can by no means be regarded as inherited in a biological sense, unless 
we suppose every metabolic anomaly to be grounded in heredity. The 
tendency ruling at present to explain many phenomena of this kind 
as the effects of hormons unfortunately prejudices the rational treat- 
ment of the problems, but it has this much importance that it under- 
lines the necessity of keeping the true, biological inheritance as distinct 
as is at all possible from other causal factors. There is every reason 
to believe, that the functions of the ductiess glands play a very im- 
portant role indeed as complicating moment, but so long as we know 
so little about the matter, it is best to consider the inheritance itself 
as the principal thing and seek for the explanation of the genetic phe- 
nomena there, especially as we must assume, that the single ductless 
glands and their functions are also subject to the general laws of 
heredity. 
It has long been recognised, that Retinitis pigmentosa occurs so fre- 
quently along with inherited deafness that there must be some genetic 
connection, but greater interest lies in the fact, that it frequently occurs 
in children of blood relations. This is not surprising however to the 
modern view of the whole problem of consanguinity, since it is a 
natural consequence of the disease being polyhybrid. Whether the 
single factors behind it are recessive or dominant, there is a greater 
probability of their meeting in endogamous than in exogamous unions, 
and that the consanguinity in itself should be able to produce or even 
favour the development of such a disease, is unlikely according to the 
present-day conception of inheritance. The disease is very rare and 
develops practically pure; without doubt it is related to a number of 
other inherited eye diseases, into which however there is no need to 
enter here. 
The great majority of the diseases occurring side by side in these 
families so weighted by inheritance, are however the mental defects in 
