Darwin devoted a large part of his life to understanding heredity. He wrote books on the subject. However, his views fluctuated with time, and historians have spent much time analysing the different ideas he entertained.
"Darwin's conclusion from his studies on inheritance was always the same, that the rules and mechanisms of inheritance were complex and not ready for a definitive analysis."
Darwin's conceptual model of evolution meant his experiments on inheritance were quite different from those of Mendel (source here)
In a helpful analysis of the issues, Jonathan Howard of the University of Cologne suggests that it is legitimate to ask why Darwin did not reach a satisfying conclusion:
"The solution, at least to the inheritance problem, was apparently easily amenable to an experimental approach with materials that were daily to hand. Furthermore the time was ripe in the middle of the 19th century, with many breeders interested in the problem for commercial as well as scientific reasons. And after all, Mendel solved the logic of inheritance in his own backyard in the monastery at Brno with no more technology than Darwin had at his disposal in his garden at Down House. Why couldn't Darwin have done it too?"
Howard looks carefully at Darwin's writings on the subject, and finds he was highly selective in his interests. He sought to document the small variations that he thought could accumulate and lead eventually to speciation. His writings reflect his commitment to this concept: "the selection of infinitesimal varieties", "differences absolutely inappreciable to the naked eye", "the accumulation of infinitesimally small inherited modifications". Howard makes this comment on Darwin's 1876 book The Effects of Cross and Self-Fertilisation in the Vegetable Kingdom:
"His overriding purpose was to establish that progeny produced by self-fertilization are less thrifty than the products of cross-fertilization. He rightly concentrated his analysis not on unit characters, but on the quantitative characters that fitted better with his concept of differential fitness, the attributes that, by their infinitesimal differences, determine life and death in the wild. So Darwin counted seeds, weighed and measured them, planted them and looked for their vitality. He measured growth and general thriftiness in his self-fertilized and cross-fertilized progeny. Everything he measured was a quantitative variable that under these simple experimental conditions could yield no information about inheritance at all. His experiments are overwhelming in scale and scope; they established the point that he wanted to establish beyond all doubt, but they contributed nothing relevant to our understanding of the underlying logic of inheritance."
This distinction between 'quantitative measures of variation' and 'unit characters' provides a clear contrast between the respective methodologies of Darwin and Mendel. Whilst Darwin was trying to put some substance into the word 'fitness' by measuring dimensions and weighing samples (continuous data), Mendel was counting characters (discrete data). Howard points out that, in the course of Darwin's research, he documented findings that we can now understand as Mendelian behaviour. Because he was not looking for it, Darwin did not see it.
"In one especially poignant case, working with the recessive character of radially symmetrical (peloric) flowers of Antirrhinum, Darwin came close to the kind of result that might have ended with a law of segregation. He crossed pure-breeding peloric plants with pure breeding wild types, noting the dominance of the wild type in the F1 progeny. He then established the F2 generation and obtained wild-type and peloric plants in a ratio (88:37) that Mendel (and now we) would effortlessly accept as representing 3:1. However, Darwin had other priorities and was in no way programmed to see the critical meaning in these numbers. He cites them within a sentence and they receive no further comment."
Darwin tried to present his approach to research as Baconian. He is often described as a scientist who gathered data and, by a process of induction, drew out theory that was grounded in evidence. Yet his work on inheritance was not at all like this. He designed his experiments and gathered data about breeding in order to develop and elaborate on his theory of evolution by natural selection. Other data was overlooked or dismissed as not relevant to the problem he was considering.
"The explanation, though, for why Darwin turned away from the inheritance of unit characters as a possible route to resolving the general inheritance problem was simply that he did not believe that such characters had anything to do with the kind of variations that he thought were the raw materials of evolutionary change. Such qualitative and striking variations he characterized as 'sports'. They might be useful for the breeders of fancy plants and animals, but although artificial selection of such anomalous variants could provide an analogy to evolution by natural selection, this was not the real thing. Again and again, when Darwin wrote generally about evolution, he came back to one issue, the infinitely tiny differences between individuals that confer infinitesimal advantages or disadvantages in the interminable struggle for existence. The selection of these variants, continued over hundreds of thousands of generations, was the critical process in evolution. These were the variations, and this was the inheritance, that mattered."
But how did Darwin develop this blindness? Is it a quirk of his personality - or are there influences on his thinking which need to be more clearly recognised. Howard traces this back to the influence Charles Lyell had on Darwin via his books.
"This view of biology was uniquely Darwin's and one he took over wholesale from Charles Lyell's uniformitarian geology during and after the Beagle voyage. Indeed this was by far the strongest, most important, and heuristically most productive of all the influences Darwin was subject to ("I always feel as if my books came half out of Lyell's brain"."
So Darwin was a product of his age. Through Hutton and Lyell, uniformitarianism entered geological science and became dominant for 150 years until people recognised that it was not unscientific to invoke catastrophes and discontinuities. Darwin brought this thinking into biology and the influence of uniformitarianism has had a negative impact on biological science for 150 years also! It delayed the acceptance of Mendelian genetics and steered scientists towards thinking that gradual adaptation was the only issue for understanding the history of life on Earth.
"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."
There are some really important lessons here for science students. Whilst researchers may set out with the best of intentions to implement Baconian induction, they need to realise that they bring mental constructs which govern experimental design and the interpretation of data. Even a genius cannot escape this! Darwin illustrates the negative impacts of uniformitarianism in biological science and provides some timely warnings to those who want to treat all critical evaluation of Darwin's work as subversive to science. In education, we need to encourage the critical appraisal of evolutionary theories, and Darwin's research into inheritance is a good place to start. The health and vigour of science demands it.