t are to be found between one individual and another. Instead
of looking at the individual as a whole, which is in some vague way endowed
with an individuality marking it off from its fellows, we now regard it as
an organism built up of definite characters superimposed on a basis beyond
which for the moment our analysis will not take us. We have begun to
realise that each individual has a definite architecture, and that this
architecture depends {136} primarily upon the number and variety of the
factors that existed in the two gametes that went to its building. Now most
species exhibit considerable variation and exist in a number, often very
large, of more or less well-defined varieties. How far can this great
variety be explained in terms of a comparatively small number of factors if
the number of possible forms depends upon the number of the factors which
may be present or absent?

In the simple case where the homozygous and heterozygous conditions are
indistinguishable in appearance the number of possible forms is 2, raised
to the power of the number of factors concerned. Thus where one factor is
concerned there are only 2^1 = 2 possible forms, where ten factors are
concerned there are 2^{10} = 1024 possible forms differing from one another
in at most ten and at least one character. Where the factors interact upon
one another this number will, of course, be considerably increased. If the
heterozygous form is different in appearance from the homozygous form,
there are three possible forms connected with each factor; for ten such
factors the possible number of individuals would be 3^{10} = 59,049; for
twenty such factors the possible number of different individuals would be
3^{20} = 3,486,784,401. The presence or absence of a comparatively small
number of factors in a species carries with it the possibility of an
enormous range of individual variation. But every one of these individuals
has a perfectly definite constitution which can {137} be determined in each
case by the ordinary methods of Mendelian analysis. For in every instance
the variation depends upon the presence or absence of definite factors
carried in by the gametes from whose union the individual results. And as
these factors separate out cleanly in the gametes which the individual
forms, such variations as depend upon them are transmitted strictly
according to the Mendelian scheme. Provided that the constitution of the
gametes is unchanged, the heredity of