Sir Ronald Aylmer Fisher 43Figure 1. Graphical Representation of Fisher’s Geometric Model. Adapted from
Burch and Chao [1999, 922]. X and Y are continuous phenotypes with optimum
fitness at O. D is a non-optimal point of fitness. See text for discussion. Fisher
himself does not use a diagram although he does articulate, mathematically, the
relevant properties.
By and large, the middle chapters ofThe Genetical Theoryare (mainly theo-
retical) explorations of cases, such as dominance, sexual selection, and mimicry,
to support and extend the preceding theoretical work. Nevertheless, in the fourth
and fifth chapters, Fisher expands his theoretical discussion to more general is-
sues concerning the causes of genetic variation. The last five chapters of the book
explore natural selection in human populations, particularly social selection in
human fertility. Fisher’s central observation, based upon England’s 1911 census
data, was that the development of economies in human societies structures the
birth-rate so that it is inverted with respect to social class. In the final chapter
of his book, Fisher offers strategies for countering this effect. He proposed the
abolishment of the economic advantage of small families by instituting what he
called “allowances” to families with larger numbers of children, with the allowances
proportional to the earnings of the father. In spite of Fisher’s espousal of eugenics
in this part of the book, he means the discussion to be taken as an inseparable
extension of the preceding part.
Fisher compared both his 1922 and 1930 exploration of the balance of evolu-
tionary factors and the “laws” that describe them to the theory of gases and the
second law of thermodynamics, respectively. Of the 1922 investigation, Fisher
says,
[t]he investigation of natural selection may be compared to the analytic
treatment of the Theory of Gases, in which it is possible to make the