Fig. 3. Mendel’s annotations on page 478 of his German translation of Darwin’s The Variation of Animals and Plants Under Domestication.

The segment marked by Mendel states (in Darwin’s original English, underlining corresponding to Mendel’s underlined words): “even thirty grains did not fertilise a single seed; but when forty grains were applied to the stigma, a few seeds of small size were formed”. Mendel also underlined “Naudin”, who, as explained by Darwin, observed that multiple pollen grains were required for fertilisation in Mirabilis (see SIF1 entry 11 for the full English text of this passage). This annotation is especially important because it prompted Mendel, in spite of eye strain, to repeat Naudin’s experiments, recounting them in a letter to Nägeli. Mendel discovered that in repeated cases one pollen grain was, in fact, sufficient for fertilisation, consistent with his theory of heredity.