The year is established by the relationship between this letter and the letter from Francis Galton, 19 December 1875.

Galton had sent CD proofs of a paper on heredity (see letter from Francis Galton, 5 November 1875), but George Howard Darwin had to explain the differences between Galton’s views and CD’s hypothesis of pangenesis (see n. 3, below) because CD had failed to understand on which points they disagreed (see letter to Francis Galton, 7 November [1875]).

CD’s revised hypothesis of pangenesis was published in Variation 2d ed. 2: 349–99. He postulated that heredity occurred through gemmules (minute granules shed by the different parts of an organism and dispersed throughout its system) that were capable, through either sexual or asexual transmission, of generating new individuals by developing into parts like those from which they were orginally derived. Many gemmules, CD held, were transmitted in a dormant state over several generations before developing in an individual (ibid., p. 374). He referred to the multiplication of gemmules in the reproductive organs in ibid., p. 379. Galton, however, argued that pangenesis accounted only for the transmission of acquired characteristics, and that the inheritance of peculiarities that were ancestral, and that might not be present in the parents, required the existence of gemmules or ‘germs’ that could not have derived from the organs of the parents. This form of heredity, he theorised, depended on the number and variety of germs (what he called the stirp) present in the sexual elements and in buds being greater than the number of organic units of the individual derived from them, and on the capacity of those germs that did not develop in an individual to retain their vitality and potential for development when they became part of the stirp of the next generation. Galton claimed that his theory explained not only how ancestral characteristics could skip generations but also the differences between offspring of the same parents. See Galton 1875b, pp. 81–3 and p. 86.

Compound ascidians (sea squirts) consist of many small individuals forming a colony through asexual reproduction, but they also reproduce sexually. Galton, however, supposed that the more complex an organism, the less likely it would be that all the germs that composed a stirp would congregate in multiple places to allow asexual budding; instead they would be concentrated in the sexual organs (see letter from Francis Galton to G. H. Darwin, 22 December 1875).