their parents. Thirdly,
squinting is a familiar example of hereditary transmission: it is
frequently a result of such optical defects as have been above
mentioned; but the more primary and uncomplicated forms of it are also
sometimes in a marked degree transmitted in a family. Fourthly,
_Cataract_, or opacity of the crystalline lens, is commonly observed in
persons whose parents have been similarly affected, and often at an
earlier age in the children than in the parents. Occasionally more than
one child in a family is thus afflicted, one of whose parents or other
relation presents the senile form of the complaint. When cataract
affects several members of a family in the same generation, it is often
seen to commence at about the same age in each; _e.g._, in one family
several infants or young persons may suffer from it; in another,
several persons of middle age. Mr. Bowman also informs me that he has
occasionally seen, in several members of the same family, various
defects in either the right or left eye; and Mr. White Cooper has often
seen peculiarities of vision confined to one eye reappearing in the
same eye in the offspring.[17]

The following cases are taken from an able paper by Mr. W. Sedgwick,
and from Dr. Prosper Lucas.[18] Amaurosis, either congenital or coming
on late in life, and causing total blindness, is often inherited; it
has been observed in three successive generations. Congenital absence
of the iris has likewise been transmitted for three generations, a
cleft-iris for four generations, being limited in this latter case to
the males of the family. Opacity of the cornea and congenital smallness
of the eyes have been inherited. Portal records a curious case, in
which a father and two sons were rendered blind, whenever the head was
bent downwards, apparently owing to the crystalline lens, with its
capsule, slipping through an unusually large pupil into the anterior
chamber of the eye. Day-blindness, or imperfect vision under a bright
light, is inherited, as is night-blindness, or an incapacity to see
except under a strong light: a case has been recorded, by M. Cunier, of
this latter defect having affected eighty-five members of the same
family during six generations. The singular incapacity of
distinguishing colours, which has been called _Daltonism_, is
notoriously