The date is established by the reference to Falconer 1857, and the reference to CD and Falconer having met (see n. 2, below). In 1857, 7 March was a Saturday.

The reference is to the manuscript of Falconer 1857; the paragraph referred to probably corresponds to that on page 276 of the published paper (see n. 3, below). Falconer’s paper was read before the Geological Society of London on 11 March 1857; it was published later in the year. According to Emma Darwin’s diary (DAR 242), CD visited London from 4 to 7 March 1857; he apparently saw Falconer during his visit (see Correspondence vol. 6, letter to J. D. Dana, 5 April [1857] and n. 8).

In his published paper (Falconer 1857, p. 276), Falconer suggested that Plagiaulax, a fossil marsupial found in Purbeck, Dorset, furnished a ‘crucial test’ of the generalisation made by Richard Owen, William Benjamin Carpenter, and others, that the degree of specialisation within a group such as the Mammalia increased over geological time. In particular, he noted, these naturalists argued that the earliest Eocene mammals usually possessed the full complement of teeth, while forms characteristic of later times were ‘remarkable for the special suppression of these organs’. In opposition to this view, Falconer argued that, while Plagiaulax was ‘the oldest well-ascertained herbivorous mammal yet discovered’, it represented ‘the most specialized exception … from the rule to be met with in the whole range of the Marsupialia, fossil or recent’. CD evidently thought that Owen might distinguish between placental mammals, to which his rule definitely applied, and marsupials (see n. 5, below).

CD refers to Owen’s classification of the Mammalia, in which the Gyrencephala included the placental mammals with more highly developed brains, except for humans (R. Owen 1857, p. 19). Owen’s classification was announced in a paper read before the Linnean Society on 17 February and 21 April 1857; CD’s letter to J. D. Dana, 5 April [1857] (Correspondence vol. 6), suggests that he had doubts about classifications based upon a single feature, in this case, brain structure.

In R. Owen 1852, cited in Falconer 1857, p. 276, Owen wrote (pp. 904–5): Examples of the typical dentition are exceptions in the actual creation; but it was the rule in the forms of Mammalia first introduced into this planet.... The Chæropotamus … and other ancient (eocene and miocene) tertiary mammalian genera presented the forty-four teeth, in number and kind according to that which is here propounded as the typical or normal dentition of the placental Mammalia. R. Owen 1850, also cited by Falconer, is similarly ambiguous about whether the progressive specialisation of the typical mammalian dentition applies to marsupials or not: see p. 495.