hologic phenomena that
there is little temptation for the advocates of Lamarckianism to use
them as proofs of their theory.]

We have hitherto been considering the results worked out by Mendel with
but one pair of contrasted characters or factors. But Mendel studied the
relation of other characters of the pea, and found among other results
that smooth seeds are dominant to wrinkled seeds, colored seeds dominant
to white, yellow color dominant to green, etc. But when a combination of
_two_ factors in each parent are put into contrast by cross breeding,
two wholly original forms (as they seemed) were sometimes produced, and
it looked as if these new kinds were really analogous to new species.

For example, he crossed tall yellow peas with dwarf green peas, with
the result that the first hybrid generation turned out to be all tall
yellows. However, in the second hybrid generation they split up
according to the law as already stated, modified by the additional
complication brought into the problem by the additional pair of factors.
For out of every sixteen plants there were nine tall yellows, three
_dwarf yellows_, three _tall greens_, and one dwarf green. It is evident
that these tall greens and dwarf yellows are really new forms; and
further experiments proved that they can be separated out or segregated
and grown as pure forms which thereafter breed true. Thus we have a very
important result for the breeder, for it enables him to work to a
definite aim and combine certain desirable characters into a single
form.

The term _mutation_, as already intimated, has been given to this
process of producing new varieties in this way. The kinds so produced
are termed _mutants_, and at first they were hailed by enthusiastic
scientists as "elementary species." De Vries in particular gave much
publicity to this idea; for he thought he had really produced a new kind
comparable in every respect to a true species as produced by nature
among wild plants. But the enthusiasm with which this applied result of
Mendel's Law was at first hailed by biologists has gradually subsided;
for it has been found that though these new forms will breed true under
certain conditions, they are nevertheless _cross-fertile with the
original forms_, and thus the circle can be _completed back again_ by a
return to the parent form, from which the new "species" can again be
produced at will with the same mathematical exactness as before.

III

Where then