of
agoutis among its offspring must be attributed to the presence of G in the
albino. Tested in this way the F_2 albinos were proved, as was expected, to
be of three kinds: (1) those which gave only agouti, _i.e._ which were
homozygous for G; (2) those which gave agoutis and blacks in approximately
equal numbers, _i.e._ which were heterozygous for G; and (3) those which
gave only blacks, and therefore did not contain G.

Though albinos, whether mice, rabbits, rats, or other animals, breed true
to albinism, and though albinism behaves as a simple recessive to colour,
yet albinos may be of many different sorts. There are in fact just as many
kinds of albinos as there are coloured forms--neither more nor less. And
all these different kinds of albinos may breed together, transmitting the
various colour factors according to the Mendelian scheme of inheritance,
{54} and yet the visible result will be nothing but albinos. Under the mask
of albinism is all the while occurring that segregation of the different
colour factors which would result in all the varieties of coloured forms,
if only the essential factor for colour development were present. But put
in the developer by crossing with a pure coloured form and their variety of
constitution can then at last become manifest.

So far we have dealt with cases in which the production of a character is
dependent upon the interaction of two factors. But it may be that some
characters require the simultaneous presence of a greater number of factors
for their manifestation, and the experiments of Miss Saunders have shown
that there is a character in stocks which is unable to appear except
through the interaction of three distinct factors. Coloured stocks may be
either hoary, with the leaves and stem covered by small hairs, or they may
lack the hairy covering, in which case they are termed glabrous. Hoariness
is dominant to glabrousness; that is to say, there is a definite factor
which can turn the glabrous into a hoary plant when it is present. But in
families where coloured and white stocks occur the white are always
glabrous, while the coloured plants may or may not be hoary. Now colour in
the stock as in the sweet pea has been proved to be dependent upon the
interaction of two separate factors. Hence hoariness depends upon three
separate factors, and a stock cannot be hoary unless {55} it contains the
hoary factor in addition to the two colour factors. It requires the
presence of