that cells, before their conversion into completely passive or “formed material” throw off minute granules or atoms, which circulate freely through the system, and when supplied with proper nutriment multiply by self-division, subsequently becoming developed into cells like those from which they were derived. . . . Hence, speaking strictly, it is not the reproductive elements . . . which generate new organisms, but the cells themselves throughout the body.

To Darwin, the great advantage that pangenesis offered over blending was that if some adaptive change were to occur during the lifetime of an organism, then the granules (or “gemmules,” as he called them) could take note of the change, lodge in the reproductive organs, and ensure that the change would be transmitted to the next generation. Unfortunately, pangenesis was taking heredity precisely in the opposite direction from which modern genetics was about to direct it—it is the fertilized egg that instructs the development of the entire body, not the other way around. Confused, Darwin clung to this misguided theory with similar conviction to that which he exhibited when he had previously held on to his correct theory of natural selection. In spite of vehement attacks by the scientific community, Darwin wrote to his great supporter Joseph Dalton Hooker in 1868: “I fully believe that each cell does actually throw off an atom or gemmule of its contents; but whether or not, this hypothesis serves as a useful connecting link for various grand classes of physiological facts, which at present stand absolutely isolated.” He also added with confidence that even “if pangenesis is now stillborn, it will, thank God, at some future time reappear, begotten by some other father, and christened by some other name.” This was a perfect example of a brilliant idea—particulate inheritance—that failed miserably because it had been incorporated into the wrong mechanism for its implementation: pangenesis.

Nowhere did Darwin articulate more clearly his atomistic, essentially Mendelian, ideas of heredity than in an exchange with Wallace in 1866. First, in a letter written on January 22, he noted, “I know of a good many varieties, which must be so called, that will not blend or intermix, but produce offspring quite like either parent.” Failing to see Darwin’s point, Wallace replied on February 4, “If you ‘know varieties that will not blend or intermix, but produce offspring quite like either parent,’ is not that the very physiological test of a species which is wanting for the complete proof of the ‘origin of species.’ ”

Realizing the misunderstanding, Darwin was quick to correct Wallace in his next letter:

I do not think you understand what I mean by the nonblending of certain varieties. It does not refer to fertility. An instance will explain. I crossed the Painted Lady and Purple sweet peas, which are very differently coloured varieties, and got, even out of the same pod, both varieties perfect, but none intermediate. Something of this kind, I should think, must occur at first with your butterflies and the three forms of Lythrum; though these cases are in appearance so wonderful, I do not know that they are really more so than every female in the world producing distinct male and female offspring.