and soles of the feet, known as
tylosis, and epidermolysis bullosa, a disease in which the skin rises up
into numerous bursting blisters.

Among the most interesting of all human pedigrees is one recently built up
by Mr. Nettleship from the records of a night-blind family living near
Monpelier in the south of France. In night-blind people the retina is
insensitive to light which falls below a certain intensity, and such people
are consequently blind in failing daylight or in moonlight. As the
Monpelier case had excited interest for some time, the records are
unusually complete. They commence with a certain Jean Nougaret, who was
born in 1637, and suffered from night-blindness, and they end for the
present with children who are to-day but a few years of age. Particulars
are known of over 2000 of the descendants of Jean Nougaret. Through ten
generations and nearly three centuries the affection has behaved as a
Mendelian dominant, and there is no sign that long-continued marriage with
folk of normal vision has produced any amelioration of the night-blind
state. {175}

[Illustration: FIG. 35.

Pedigree of a haemophilic family. Affected (all males) represented by
black, and normals of both sexes by light circles. (From Stahel.)]

Besides cases such as these where a simple form of Mendelian inheritance is
obviously indicated, there are others which are more difficult to read. Of
some it may be said that on the whole the peculiarity behaves as though it
were an ordinary dominant; but that exceptions occur in which affected
children are born to unaffected parents. It is not impossible that the
condition may, like colour in the sweet pea, depend upon the presence or
absence of more than one factor. In none of these cases, however, are the
data sufficient for determining with certainty whether this is so or not.

A group of cases of exceptional interest is that in which the incidence of
disease is largely, if not absolutely, restricted to one sex, and so far as
is hitherto known the burden is invariably borne by the male. In the
inheritance of colour-blindness (p. 117) we have already discussed an
instance in which the defect is rare, though not {176} unknown, in the
female. Sex-limited inheritance of a similar nature is known for one or two
ocular defects, and for several diseases of the nervous system. In the
peculiarly male disease known as haemophilia the blood refuses to clot when
shed, and there is nothing to prevent g