Darwin is occasionally criticized as an imprecise, non-numeric naturalist, a man of ideas, perhaps brilliant and original in that mode, but not a scientist like those of today. Indeed I am sure I have heard this view from eminent members of my own institute. Mendel's rational, experimental analysis of the inheritance of unit characters is without question a work of great genius. One reads it still with the same sense of breathless excitement, of irresistible intellectual force and forward propulsion that one experiences, for example, from the extraordinary Nature
paper by Crick, Brenner, Barnett and Watts-Tobin on the encoding of proteins [10
]. However, if Darwin failed to discover Mendel's laws, it was not so much because of what he lacked in genius or numeracy or the experimental cast of mind, but rather because of the forceful tendency of what he already possessed. His focus on continuous variation as the source of evolutionary change was not wrong, and coupled with the power he could see in the integration of infinitesimals over time he built his case on the solid foundation of Lyell's uniformitarian thinking. Much of variation and inheritance was simply opaque in those terms, but continuous variation, not unit characters, was, for Darwin, the way forward. Thus Darwin boxed himself in, unable to see the laws of inheritance in continuous variation, unable to see the real importance of discontinuous variation where the laws of inheritance could be discerned.
The scientist in the best position to resolve Darwin's problems was his own cousin, Francis Galton (Figure ). While Darwin was wrestling with pangenesis Galton was pioneering the analysis of the inheritance of quantitative characters and he documented with extraordinary insight the properties of such inheritance. In particular, he documented the phenomenon of regression to the mean in the context of the inheritance of quantitative characters such as height or intelligence. Regression to the mean records the interaction between control of the character by multiple polymorphic loci of small effect and, of course, multiple environmental effects. Galton approached a correct genetic interpretation of this phenomenon while Darwin confused it with blending inheritance. He also took a lively interest in pangenesis (which he eventually rejected, to Darwin's chagrin) and corresponded extensively with Darwin about it. In a somewhat gnomic response to a query from Darwin, Galton replied "If there were two gemmules only, each of which might be white or black, then in a large number of cases one-quarter would always be quite white, one-quarter quite black, and one half would be grey". Such is indeed the stuff of continuous variation seen from a Mendelian perspective, but perhaps Galton was too distant from the biology of inheritance and too interested in the mathematics of what he was discovering to take his own suggestion to its most important conclusion.
Figure 7 Francis Galton, the anthropometrician, in 1893. Reproduced from an anthropometric study cited in .
Galton died in 1911, a full decade after the multiple republications of Mendel's paper and multiple reiterations of his findings by other authors. It is surprising that Galton in his old age did not take the opportunity, which he of all geneticists of that era was qualified to take, to reconcile the Mendelian laws of inheritance with Darwin's (and his own) priority for continuous variation as the fuel of evolutionary change. In the event, most of Galton's contemporaries largely abandoned Darwin's view in favor of mutations of large effect as the driving force for evolutionary change. William Bateson, in his 1894 Materials for the Study of Variation
], had already backed large-scale discontinuous variation as the basis for evolution and the origin of species, "for if distinct and 'perfect' varieties may come into existence discontinuously, may not the Discontinuity of Species have had a similar origin? If we accept the postulate of Common Descent this expectation is hard to resist. In accepting that postulate it was admitted that the definiteness and Discontinuity of Species depends upon the greater permanence of certain terms in the series of Descent. The evidence of Variation suggests that this greater stability depends primarily not on a relation between organism and environment, not, that is to say, on Adaptation, but on the Discontinuity of Variation. It suggests, in brief, that the Discontinuity of Species results from the Discontinuity of Variation." Elsewhere, the sneering tone of Bateson's concluding remarks on adherents of the Darwinian view suggests the ill-humor that accompanied this debate right through until the late 1920s, when Fisher, Haldane, Sewall Wright and others reconciled Mendelism with continuous variation (reviewed in [12
]). Hugo de Vries, whose own studies of discontinuous variation led him to the rediscovery of Mendel, also argued [13
] for variation of large effect as the driving force in evolution, and even as late as the 1930s this strand of thought was still prominent in Richard Goldschmidt's 'hopeful monsters' [14
]. The old dichotomy between continuous and discontinuous variation as the engines of evolution surfaced again in a slightly different form in our own generation through the polemic of Niles Eldredge and Stephen J Gould for evolution by punctuated equilibrium [15
]. No doubt we shall see it again.