64 Sahotra Sarkar
would finally codify his ideas into the three-phase “shifting balance theory” (see
below).^25 In sharp contrast, the “Mathematical Theory” appears almost mun-
dane. Models of selection are introduced, their virtues and vices discussed, and
then almost cast aside as other models are explored. The few general claims, most
notably, the skepticism about the power of selection to spread recessive genes and
the possible importance of mutations, are carefully presented with many caveats
and qualms. Haldane carefully discusses the known experimental data, and was
more concerned with connecting models to data than either Wright or Fisher were
during this period. As he pointed out, himself, Part I included a discussion of “all
the then known types of single-factor Mendelian inheritance” [1927b, 843]. How-
ever, the “Mathematical Theory” does not even contain a systematic discussion of
the relevance of the models to all the experimental results that had accumulated
during this period. The latter lacuna, at least, was removed in theCauses of
Evolution.
4 THE CAUSES OF EVOLUTIONIn January 1931 Haldane delivered a series of lectures at the Prifysgol Cymru
at Aberystwyth, Wales, entitled “A Re-examination of Darwinism”. The text of
these, along with an Appendix, became Haldane’s best-known book,The Causes
of Evolution.^26 Haldane started with the unquestionable fact of evolution, “the
descent from living beings in the past of other widely different living beings” [1932,
4]. All that remained debatable were its possible causes.
The first chapter considered five causes that had been historically suggested
“for the deeper transformations of the geological record” [1932, 11]: (a) basically
random inheritable variation; (b) environmentally induced inheritable variation;
(c) variation due to “internal causes”; (d) variation due to hybridization; and (e)
selection. Though Darwin had coupled selection with both random and acquired
inheritable variation, experimental evidence had largely ruled out the latter. Those
who believed in natural selection as a major cause of evolution were thus left
with selection acting on random (or blind) inheritable variation. Haldane’s series
of papers had provided a quantitative basis for neo-Darwinism. It remained to
provide a full biological interpretation of the new theory, to show how the new
genetics permitted the construction of a complete theory of evolution from only
causes (a) and (e). This was the major contribution ofCauses.
By the late 1920’s, the so-called Lamarckists who advocated the inheritance
of acquired characters were rare. InCauses, Lamarckism is dismissed because of
its experimental failures (pp. 130–138). The other alternatives to neo-Darwinism
(^25) See Crow [1990] for a particularly lucid introduction to the shifting balance theory. It is
reproduced in Sarkar [1992].
(^26) In spite of its reputation for being well-written,Causesis hard to follow. Arguments are
often imprecise and the narrative disjointed. It reads like a transcript of lectures that could have
profited from more careful editing. One of the most humorous of the editorial lapses occurs in
the list of references where all of Haldane’s works are attributed to Charlotte.