Darwin subscribed to the now-discredited model of blending inheritance, in which traits in the offspring resemble the average of those in the parents. For example, this model predicts that the offspring of plants with blue flowers and those with red flowers will have purple flowers. While traits of offspring are sometimes the average of those of the parents (think of certain cases of human height), the idea of blending inheritance—which implies the blending of the genetic material—as a general rule presents problems. For example, it cannot explain the reappearance of a trait several generations after it apparently “disappeared” in a family, such as red hair or blue eyes.

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Another difficulty with the concept of blending inheritance is that variation is lost over time. Consider an example where black rabbits are rare and white rabbits are common (Fig. 16.1). Black rabbits, being rare, are most likely to mate with the much more common white rabbits. If blending inheritance occurs, the result will be gray rabbits. These will also mate with white rabbits, producing lighter-gray rabbits. Over time, the population will end up being all white, or very close to white, and there will be less variation. The only way that black rabbits will be present is if they are reintroduced into the population through mutation or migration.

Whatever the trait may be, blending inheritance predicts that inheritance will tend to be a homogenizing force, producing in each generation a blend of the original phenotypes. But we know from common experience that variation in most populations is plentiful.

It is ironic that Darwin believed in blending inheritance because this mechanism of inheritance is incompatible with his theory of evolution by means of natural selection. The incompatibility was pointed out by some of Darwin’s contemporaries, and Darwin himself recognized it as a serious problem. The problem with blending inheritance is that rare variants, such as the black rabbits in the above example, will have no opportunity to increase in frequency, even if they survive and reproduce more than white rabbits, since they gradually disappear over time.

Although Darwin was convinced that he was right about natural selection, he was never able to reconcile his theory with the concept of blending inheritance. Unknown to him, the solution had already been discovered in experiments carried out by Gregor Mendel (1822–1884), an Augustinian monk in a monastery in the city of Brno in what is now the Czech Republic. Mendel’s key discovery was this: It is not traits that are transmitted in inheritance—it is genes that are transmitted.