Darwin could not be faulted for not knowing any better than the heredity theory that was scientifically accepted at his time. Consequently, I do not consider his adopting the idea of blending inheritance as a blunder. Darwin blundered in having completely missed the point (at least initially) that his mechanism of natural selection simply could not work as envisioned, under the assumption of blending inheritance. Let us examine this serious blunder and its potentially devastating consequences in more detail.

Fleeming Jenkin published his criticism of Darwin’s theory as an anonymous review of the fourth edition of On the Origin of Species. The article appeared in the North British Review in June 1867. While the essay attacked the theory of evolution on several grounds, I shall concentrate here on the one argument that exposed Darwin’s blunder. To demonstrate his point, Jenkin assumed that each individual has one hundred offspring, but of those, on the average, only one survives to reproduce. He then discussed an individual with a rare mutation (“sport”) that has the advantage of having twice the chance of survival and reproduction as any other. Appropriately for the rigorous engineer that he was (he received no fewer than thirty-seven patents between 1860 and 1886), Jenkin’s approach was quantitative—he wanted to actually calculate the effect of such a “sport” on the general population:

It will breed and have a progeny of say 100; now this progeny will, on the whole, be intermediate between the average individual and the sport. [Since the sports are rare, a sport is expected to mate with an average individual.] The odds in favour of one of this generation of the new breed will be, say, 1.5 to 1 [under the assumption of blending], as compared with the average individual; the odds in their favor will therefore be less than that of their parent; but owing to their greater number, the chances are that about 1.5 of them would survive. Unless these breed together, a most improbable event, their progeny would again approach the average individual; there would be 150 of them [1.5 times 100], and their superiority would be say in the ratio of 1.25 to 1 [again because of blending]; the probability would now be that nearly two of them would survive [1 percent of 1.25 times 150] and have 200 children, with an eighth superiority. Rather more than two of these would survive; but the superiority would again dwindle, until after a few generations it would no longer be observed, and would count no more in the struggle for life, than any of the hundred trifling advantages which occur in the ordinary organs.

Jenkin argued that even under the most extreme form of selection, one could not expect the complete transformation of a well-established characteristic, such as skin color, into a new one, if that new characteristic had been introduced into the population only once. To illustrate this swamping effect, Jenkin chose a startlingly prejudicial example of a white man with superior characteristics shipwrecked on an island inhabited by blacks. The racist and imperialistic tone of the passage utterly shocks us today, but it probably was commonplace in late-Victorian Britain: Even if this person “would kill many blacks in the struggle for existence” and “would have a great many wives and children,” and “in the first generation there will be some dozens of intelligent young mulattoes,” Jenkin argued, “can any one believe that the whole island will gradually acquire a white, or even a yellow population?”

As it turned out, Jenkin actually made one serious logical mistake in his calculations. He assumed that each sexual pair had one hundred offspring, of whom, on the average, only one offspring survived to reproduce. However, since only females can reproduce, it follows that out of each mating couple, two offspring must on the average survive (one male and one female); otherwise the size of the population would be halved in each generation—a recipe for rapid extinction. Surprisingly, only Arthur Sladen Davis, an assistant mathematics master at Leeds Grammar School, discovered this obvious error, and he explained it in a letter to the journal Nature in 1871.