y the colour-blind male are of two sorts only, viz. Mfx and mfX.
If he marries a normal woman (Ffmmxx), the spermatozoa Mfx unite with ova
fmx to give normal males, while the spermatozoa mfX unite with ova Fmx to
give females which are heterozygous for the colour-blind factor. These
daughters are themselves normal, but transmit the condition to about half
their sons. {119}

The attempt to discover a simple explanation of the nature of sex has led
us to assume that certain combinations between gametes are incapable of
giving rise to zygotes which can develop further. In the various cases
hitherto considered there is no reason to suppose that anything of the sort
occurs, or that the different gametes are otherwise than completely fertile
one with another. One peculiar case, however, has been known for several
years in which some of the gametes are apparently incapable of uniting to
produce offspring. Yellow in the mouse is dominant to agouti, but hitherto
a homozygous yellow has never been met with. The yellows from families
where only yellows and agoutis occur produce, when bred together, yellows
and agoutis in the ratio 2 : 1. If it were an ordinary Mendelian case the
ratio should be 3 : 1, and one out of every three yellows so bred should be
homozygous and give only yellows when crossed with agouti. But Cuenot and
others have shown that _all_ of the yellows are heterozygous, and when
crossed with agoutis give both yellows and agoutis. We are led, therefore,
to suppose that an ovum carrying the yellow factor is unproductive if
fertilised by a spermatozoon which also bears this factor. In this way
alone does it seem possible to explain the deficiency of yellows and the
absence of homozygous ones in the families arising from the mating of
yellows together. At present, however, it remains the only definite
instance among animals in which we have {120} grounds for assuming that
anything in the nature of unproductive fertilisation takes place.[8]

If we turn from animals to plants we find a more complicated state of
affairs. Generally speaking, the higher plants are hermaphrodite, both
ovules and pollen grains occurring on the same flower. Some plants, however
like most animals, are of separate sexes, a single plant bearing only male
or female flowers. In other plants the separate flowers are either male or
female, though both are borne on the same individual. In others, again, the
conditions are even more complex, for the sa