reat loss from even a superficial
scratch. In its general trend the inheritance of haemophilia is not unlike
that of horns among sheep, and it is possible that we are here again
dealing with a character which is dominant in one sex and recessive in the
other. But the evidence so far collected points to a difference somewhere,
for in haemophilic families the affected males, instead of being equal in
number to the unaffected, show a considerable preponderance. The
unfortunate nature of the defect, however, forces us to rely for our
interpretation almost entirely upon the families produced by the unaffected
females who can transmit it. Our knowledge of the offspring of "bleeding"
males is as yet far too scanty, and until it is improved, or until we can
find some parallel case in animals or plants, the precise scheme of
inheritance for haemophilia must remain undecided.

Though by far the greater part of the human evidence relates to abnormal or
diseased conditions, a start has been made in obtaining pedigrees of normal
characters. From the ease with which it can be observed, it was natural
that eye-colour should be early selected as a subject of investigation, and
the work of Hurst and others {177} has clearly demonstrated the existence
of one Mendelian factor in operation here. Eyes are of many colours, and
the colour depends upon the pigment in the iris. Some eyes have pigment on
both sides of the iris--on the side that faces the retina as well as on the
side that looks out upon the world. Other eyes have pigment on the retinal
side only. To this class belong the blues and clear greys; while the eyes
with pigment in front of the iris also are brown, hazel, or green in
various shades according to the amount of pigment present. In albino
animals the pigment is entirely absent, and as the little blood-vessels are
not obscured the iris takes on its characteristic pinkish-red appearance.
The condition in which pigment is present in front of the iris is dominant
to that in which it is absent. Greens, browns, or hazels mated together
may, if heterozygous, give the recessive blue, but no individuals of the
brown class are to be looked for among the offspring of blues mated
together. The blues, however, may carry factors which are capable of
modifying the brown. Just as the pale pink-tinged sweet pea (Pl. IV., 9)
when mated with a suitable white gives only deep purples, so an eye with
very little brown pigment mated with certain bl