Seeds of Hierarchy 207
explanation of degeneration. Panmixia is a genuine, but weak, force; it can reduce
the average value of an organ to a state somewhat below its former functional size.
But panmixia cannot solve the central question of degeneration: what propels a
useless organ all the way down the slide and into history's dumpster? Weismann
admitted his failure (1896, p. 22), and later summarized this ultimately
unsuccessful episode in his quest to understand degeneration:
As my doubts regarding the Lamarckian principle grew greater and greater,
I was obliged to seek for some other factor in modification, which should
be sufficient to effect the degeneration of a disused part, and for a time I
thought I found this in panmixia, that is, in the mingling of all together,
well and less well equipped alike. This factor does certainly operate, but the
more I thought over it the clearer it became to me that there must be some
other factor at work as well, for while panmixia might explain the
deterioration of an organ, it could not explain its decrease in size, its
gradual wearing away, and ultimate total disappearance. Yet this is the path
followed, slowly indeed, but quite surely, by all organs, which have
become useless (1903, vol. 2, p. 115).Weismann therefore needed another kind of auxiliary hypothesis to preserve
the Allmacht of selection against resurgent Lamarckism. He had tried the
mechanics of inheritance as expressed in the doctrine of panmixia; now he would
expand the domain of selection itself. He would depart from Darwin's distinctive
focus on struggle among organisms, and attempt to identify a source of directional
variation in an analogous competition among determinants of heredity within germ
cells—a "germinal selection." Weismann devised a truly ingenious argument: if
natural selection can produce trends in the morphology of phenotypes, then an
intracellular, germinal selection might yield directionality in the variation
presented to conventional selection upon organisms. If the determinants of a
useless organ predictably lose in an intracellular struggle for existence, then a trend
to complete elimination—an apparent example of Lamarckian inheritance by the
principle of disuse—might still be attributed to selection. This new mechanism
could not be equated with Darwinian selection upon struggling organisms, but
"germinal selection" did represent a process of the same form and logic, but
applied to replicating objects at a subcellular scale rather than to entire organisms.
Weismann first proposed the theory of germinal selection as a brief note in his
last rebuttal to Herbert Spencer, thus marking Britain's Victorian pundit as a chief
source (in reaction) to the first explicit theory of hierarchical selection. (Neue
Gedanken zur Vererbungsfrage, eine Antwort an Herbert Spencer, Jena, 1895).
Weismann then elaborated the theory in 1896 (presented to the International
Congress of Zoology at Leiden on September 16, 1895; first published in The
Monist in January, 1896, then as a separate pamphlet, translated into English later
that year). Weismann's fullest development, with some remarkable changes by
extension, appeared in his most important book, Vortrage iiber Descendenztheorie
(1902), translated into English