67.]
We need not be particularly concerned here with the theoretical
explanations of these facts offered in terms of the microscopic or even
the infra-microscopic components of the germ cells. Morgan seems to
make out a strong case for the theory that the chromosomes found in the
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.
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