Figure 5

But under Mendelian heredity (figure 6), since the A gene is preserved from one generation to the next, eventually two Aa’s will mate and produce the black AA variety. If black confers an advantage in the environment, then given enough time, natural selection could even turn the entire population black.

The conclusion is simple: For Darwin’s theory of evolution by natural selection to really work, it needed Mendelian heredity. But in the absence of yet-undiscovered genetics, how did Darwin respond to Jenkin’s criticism?

Darwin was a genius in many ways, but he definitely was not a sharp mathematician. In his autobiography, he acknowledged, “I attempted mathematics, and even went during the summer of 1828 with a private tutor (a very dull man) to Barmouth, but I got on very slowly. The work was repugnant to me, chiefly from my not being able to see any meaning in the early steps of algebra . . . I do not believe that I should ever have succeeded beyond a very low grade.” That being the case, arguments in The Origin are generally qualitative rather than quantitative, especially when it comes to the production of evolutionary change. In the few places where Darwin attempted to do simple calculations in The Origin, he managed occasionally to botch them. No wonder, then, that in one of his letters to Wallace, after reading Jenkin’s rather mathematical criticism, he confessed, “I was blind and thought that single variations might be preserved much oftener than I now see is possible or probable.” Still, it would have been amazing to think that Darwin had been totally unaware of the potential swamping effect of blending heredity until he read Jenkin’s article. And indeed he wasn’t. As early as 1842, twenty-five years before the publication of Jenkin’s review, Darwin had already observed, “If in any country or district all animals of one species be allowed freely to cross, any small tendency in them to vary will be constantly counteracted.” In reality, Darwin even relied to some extent on swamping to produce populational integrity in the face of the tendency of individuals to depart from their type due to variations. How did he then fail to understand how difficult it would be for a “sport” (a single variation) to fight off the averaging force of blending? Darwin’s blunder and his slowness to recognize the point raised by Jenkin probably reflected on one hand his conceptual difficulties with heredity in general, and on the other, his residual overattachment to the idea that variations had to be scarce. The latter may have partially been a consequence of his general theory of reproduction and development, in which he assumed that only developmental stress triggered variations. Darwin’s bafflement with heredity ran much deeper, as can be seen from the following inconsistency. At one point in The Origin, Darwin noted:

When a character which has been lost in a breed, reappears after a great number of generations, the most probable hypothesis is, not that the offspring suddenly take after an ancestor some hundred generations distant, but that in each successive generation there has been a tendency to reproduce the character in question, which at last, under unknown favourable conditions, gains an ascendancy.