The Development of Population Genetics 321particular organs of the offspring could be calculated from those of the parents and
those of the general population, but large numbers of families of similar parents
in that population were required before the prediction was accurate. Mendelism
dealt with natural pairs of unit characters and biometry with measurements on
samples to which one then applied statistical methods. To that extent one might
consider biometrical methods as artificially measuring features of a population
while Mendelism dealt with natural traits.
Fisher criticized the Mendelians for their preoccupation with detail and an
accompanying lack of attention to abstract reasoning — “[a] blindness to well-
established, and as far as their own facts are concerned, coordinating principles”
[1915, 59]. He claimed that it is in the “highest degree unlikely” that Mendelism
will ever cover even the field of heredity, and any amount of information about
the mechanism of heredity cannot do awaywith the need for broad co-ordinating
principles of evolution [Ibid., 60]. By contrast, Darwin’s work did “cover” the
field insofar as it continued to explain and coordinate new facts (the required
scientific test). It was in the context of this 1915 work that Fisher introduced an
idea that would have an important influence on his thinking about populations
and their evolutionary development. The comparison was between populations
construed in terms of of Mendelian factors (genes) and the populations of molecules
that constitute a gas. The agencies of selection always act amidst a multitude of
random causes, each of which may have a predominant influence if we fix our
attention on a particular individual. Yet, these agencies determine the progress
or decline of the population as a whole. In the case of the kinetic theory we have
molecules moving freely in all directions with varying velocities, yet we can obtain
a statistical result that is a perfectly definite measurable pressure. Knowledge
of the nature and properties of the atom is inessential and independent of our
knowledge of general principles in the way that our ability to predict and control
the way populations evolve is independent of particular knowledge of individuals.
This analogical model would loom large in Fisher’s later work, informing the way
in which biological populations should be conceived of in order to produce the
appropriate kinds of statistical results.
The first step toward a reconciliation of biometry and Mendelism was in 1918
in a paper whose stated goal was the investigation of biometrical properties of a
population of a “more general type” than was usually considered, in the hopes that
a more exact analysis of the causes of human variability might be given. Fisher
began by introducing the elementary statistical concepts involved in measuring
variability within a population. Basically if we take a population of roughly 1000
individuals and measure stature, the measurements are usually grouped symmet-
rically around a mean value, which is the average stature of the sample. The
distribution of deviations from this average follows the law of errors allowing us to
measure variability as one would measure errors of observation, by the mean of the
squares of the deviations of different individuals from the mean. And, as Fisher
notes, this mean square deviation is comparable to the mean square error used
in the physical sciences. This mean square deviation Fisher called the variance,