nucleus are the real ultimate units that carry the hereditary factors.
But he is quite decided in the opinion that these hereditary factors are
fixed, and are not changed from generation to generation either by
environment or by selection.[27] The important thing for us in this
connection is to get a clear idea of the results following from an
application of Mendel's laws to the old, old problem of the origin of
species, incidentally noticing how the theory associated with Darwin's
name now looks in the light of these new facts.

[Footnote 27: In human beings it has been found that the effects of
alcoholism and of syphilis are indeed transmitted according to Mendelian
law, being the two solitary examples of diseased conditions that are
thus transmitted. But they are so plainly pathologic 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.