or a little one--they will give the same result. In a word, the germ
plasm in each of these lines is pure, or homozygous, as we say. The
differences that we find between the weights (or sizes) of the individual
beans are due to external conditions to which they have been subjected.

In a word, Johannsen's work shows that the frequency distribution of a pure
line is due to factors that are extrinsic to the germ plasm. It does not
matter then which individuals in a pure line are used to breed from, for
they all carry the same germ plasm.

We can now understand more clearly how selection acting on a general
population brings about results in the direction of selection.

An individual is picked out from the population in order to get a
particular kind of germ plasm. Although the different classes of
individuals may overlap, so that one can not always judge an individual
from its appearance, nevertheless on the whole chance favors the picking
out of the kind of germ plasm sought.

In species with separate sexes there is the further difficulty that two
individuals must be chosen for each mating, and superficial examination of
them does not insure that they belong to the same group--their germ plasm
cannot be inspected. Hence selection of biparental forms is a precarious
process, now going forward, now backwards, now standing still. In time,
however, the process forward is almost certain to take place if the
selection is from a heterogeneous population. Johannsen's work was
simplified because he started with pure lines. In fact, had he not done so
his work would not have been essentially different from that of any
selection experiment of a pure race of animals or plants. Whether Johannsen
realized the importance of the condition or not is uncertain--curiously he
laid no emphasis on it in the first edition of his "Elemente der exakten
Erblichkeitslehre".

It has since been pointed out by Jennings and by Pearl that a race that
reproduces by self-fertilization as does this bean, automatically becomes
pure in all of the factors that make up its germ plasm. Since
self-fertilization is the normal process in this bean the purity of the
germ plasm already existed when Johannsen began to experiment.

HOW HAS SELECTION IN DOMESTICATED ANIMALS AND PLANTS BROUGHT ABOUT ITS
RESULTS?

If then selection does not bring about transgressive variation in a general
population, how can selection produce anything new? If it can not produce