elements capable of transposing to different chromosomal locations in maize plants (McClintock, 1945). The book contains a brief mention of Marcus Rhoades' observation that a standard recessive a₁ allele of a gene in the anthocyanin biosynthetic pathway can become unstable and revert at a high frequency to the dominant A₁ allele in a background containing a dominant Dt (“dotted”) allele (Stebbins Jr., 1950). But transposable elements were not yet common fare, nor was it known that Dt is a transposon.

Today we know that transposons constitute a large fraction—even a majority—of the DNA in some species of plants and animals, among them mice, humans, and such agriculturally important plants as corn and wheat. Given what we now know about genome organization, it is paradoxical that the discovery of transposable elements lagged so far behind the discovery of the basic laws of genetic transmission. And it is equally curious that even when they were discovered, acceptance of their generality and recognition of their ubiquity came so slowly. It is perhaps an understatement to say that McClintock's early communications describing transposition were not widely hailed for their explanatory power. Indeed, McClintock commented in the introduction to her collected papers that the response to her first effort in 1950 to communicate her discovery of transposition in “. . . a journal with wide readership . . . ,” specifically the Proceedings of the National Academy of Sciences, convinced her that “. . . the presented thesis, and evidence for it, could not be accepted by the majority of geneticists or by other biologists” (McClintock, 1987). By contrast, the explanatory power of Watson and Crick's 1953 Nature paper on the structure and mode of replication of nucleic acids was recognized immediately (Watson and Crick, 1953).

An informative parallel is provided by the contrast between the immediate recognition of the importance of Darwin's theory of evolution and the long delay between Mendel's articulation of the laws of heredity and their wide acceptance in evolutionary thinking (Carlson, 1966). It can be speculated that this was because Darwin's theory provided immediate explanations in the realm of the perceptible, whereas the hereditary mechanisms underlying variation were obscure. Variation, in Darwin's view, was continuous. Geneticists sharing his view formed the “biometrical school,” devoted to the statistical analysis of inheritance. It was not at all clear how the simple rules derived by Mendel for the hereditary behavior of “differentiating characters” bore on the problem of evolution (Carlson, 1966). The relevance of discontinuous variation or the production of “sports, ” as morphological mutations were called, was even less obvious, because the biometric approach treated offspring as statistical combinations of parental traits. Thus the idea that the study of mutations was central to understanding evolution was close to unimaginable a century ago.