Davis showed that when a correction is made to keep the population roughly constant in size, the effect of a sport does not die out (as Jenkin contended), but, in fact, although diluted, it becomes distributed throughout the entire population. For instance, a black cat introduced into a population of white cats would (under the assumption of blending inheritance) on the average produce two gray kittens, four lighter grandkittens, and so on. Successive generations would become progressively lighter, but the dark hue would never disappear. Davis also concluded correctly that “though any favourable sport occurring once, and never again, except by inheritance, will effect scarcely any change in a race, yet that sport, arising independently in different generations, though never more than once in any one generation, may effect a very considerable change.”

In spite of Jenkin’s mathematical error, his general criticism was correct: On the supposition of blending inheritance, even under the most favorable conditions, a black cat occurring once could not turn an entire population of white cats black, no matter how advantageous the black color might have been.

Before we scrutinize the question of how Darwin could have missed this seemingly fatal shortcoming of his theory of natural selection, it would be helpful to understand the blending theory of heredity from the perspective of modern genetics.

In the context of our current understanding of genetics, the molecule known as DNA (deoxyribonucleic acid) provides the mechanism responsible for heredity in all living organisms. Very roughly speaking, DNA is made up of genes, which contain the information that codes for proteins, and of some noncoding regions. Physically, DNA is located on elements called chromosomes, of which each individual organism in sexual species has two sets, one inherited from the mother (the female) and one from the father (the male). Consequently, each individual has two sets of all of its genes, where the two copies of a gene may be identical, or slightly different. The different forms of a gene that can be present at a particular location on a chromosome are the variants referred to as alleles.

The modern theory of genetics originated from the mind of an unlikely explorer: a nineteenth-century Moravian priest named Gregor Mendel. He performed a series of seemingly simple experiments in which he cross-pollinated thousands of pea plants that produce only green seeds with plants that produce only yellow seeds. To his surprise, the first offspring generation had only yellow seeds. The next generation, however, had a 3:1 ratio of yellow to green seeds. From these puzzling results, Mendel was able to distill a particulate, or atomistic, theory of heredity. In categorical contrast to blending, Mendel’s theory states that genes (which he called “factors”) are discrete entities that are not only preserved during development but also passed on absolutely unchanged to the next generation. Mendel further added that every offspring inherits one such gene (“factor”) from each parent, and that a given characteristic may not manifest itself in an offspring but can still be passed on to the following generations. These deductions, like Mendel’s experiments themselves, were nothing short of brilliant. Nobody had reached similar conclusions in almost ten thousand years of agriculture. Mendel’s results at once disposed of the notion of blending, since already in the very first offspring generation, all the seeds were not an average of the two parents.