182 D. M. Walsh
2.2 Mendelism
Mendel’s theory holds that inheritance is particulate. Even if there should be some
blending of inherited variants within a generation, the Mendelian mechanism of
inheritance allows the variant factors to be recovered and transmitted intact from
generation to generation without change. Mendel’s process was made to measure
for Darwin’s theory.^2 When combined with the Mendelian theory of inheritance,
the theory of selection tells us that adaptive evolution is the change in relative
frequency ofunchanginginheritable factors within a population, as a function of
their systematic contribution to differential survival and reproduction. Because the
factors remain unchanged through successive iterations of selection, variation —
the grist for selection — is preserved in the population for longer than it would be
under blending inheritance, and this enhances the efficacy of selection [Fisher 1930,
Ch 1]. When Darwinism is combined with Mendelism, invariant factors become
the units of inheritance, and evolution is a change in their relative frequencies.
Even so, selection has an overall tendency to eliminate variants over time. Dar-
win’s process needs a source of new variants and Mendelism, in it purest form,
is non-committal about that source. It is consistent with Mendelism that new
variants may be introduced into a population through the process of development,
and then inherited as invariant factors.^3 Nevertheless, the Mendelian process gen-
erally leaves development little or no role in determining the content of those traits
transmitted from parent to offspring.
2.3 Weissmannism
August Weissman’s experimental work demonstrated two important facts about
development. The first is that the germ plasm of animals is sequestered early in
development. It is only elements of the germ plasm that are transmitted from
parent to offspring in reproduction. Developmental differentiation takes place
(almost) exclusively in the somatoplasm. The germ plasm is quarantined from
changes wrought on the somatoplasm during an organism’s development. There
is no interchange of materials between these two lineages of cells, and very little
influence of one on the other. The second important fact is that at leastsome
changes in organismal form introduced into the somatoplasm fail to be passed on
to the following generation. Together these observations have been widely taken
to entail that the only heritable traits are those whose development is ‘encoded’ in
the germ plasm at an organism’s inception. Given that, the new variants on which
selection works must generated exclusively fromwithinthe heritable material.
Development thus has no bearing on what is inherited.
The Weissman doctrine further peripheralizes the process of development from
the explanation of fit and diversity. The fit and diversity of organisms comes about
through the process of the generation of novel variants in the heritable material,
(^2) The difficulties in getting the initial fit right, notwithstanding [Provine, 1971].
(^3) This is a conceivable position in logical space, although I’m unaware of any early 20thcentury
thinker who occupies it.