Running from High Elms in the north-east of the site, south towards the Kent Downs Area of Outstanding Natural Beauty, the Cudham Valley is a dry chalk valley parallel to the Downe Valley and divided from it by the flat plateau land on which sits Down House. Like the Downe Valley, its sides support chalk grassland, with deeper soil in the valley bottom and acid soil derived from the clay-with flints at the tops of the slopes. Some semi-natural ancient woodland remains, mainly on the sticky clay soils. In the spring they are rich in bluebells and other ancient woodland indicator species including early dog violets, wood anemones, townhall clock, yellow archangel and the delicate wood sorrel whose sleep movements were studied by Darwin.
In some places there are some semi-natural ancient woodland indicator plants within the site that are associated with chalk rather than acid soils. These include spurge laurel, early dog violet and green hellebore. The latter was noted by Charles Darwin and still grows exactly where he found it. The dormouse is also found in some of the woodlands. Dormice are fully protected because of their diminishing numbers but in Darwin’s time were more common. John Lubbock records finding one in early spring eating his crocuses at High Elms and how he took it indoors where it had four babies on the following day. Darwin also observed dormice on the ground. In a letter dated 21st September 1878 from Emma Darwin to her son William, she wrote:
"F[ather] always sits at our window [probably bedroom] early in th mg. & saw an odd sight today – a little brown creature darting across the lawn pursued by blackbirds & thrushes – proving to be a Dormouse & a short time afterwards a weasel trying to follow up the scent also pursued by birds. F. was able to see that he had quite lost the scent – but he was puzzled why the birds pursued the innocent Dormouse."
Darwin carried out many studies of plants and animals in the Cudham Valley as he investigated the driving force behind his theory of evolution, natural selection, but two areas were of special importance: Downe Bank and High Elms. These are dealt with separately, but Darwin also recorded observations in other parts of the valley. In particular he studied great mullein (Verbascum thapsus), white mullein (Verbascum lychnitis) and hybrids between them.
"Oct 4' /62/ I searched field in Cudham valley & found dozens of the yellow Verbascum, [like] lychnitis, & examined many capsules:- all utterly sterile - These plants resemble V. lychnitis but are <> taller: they all have bright yellow flowers, which are flat & remain open like V. L. & in this respect differ from V. thapsus. The anthers of longer stamens are transverse like those of V. L. whereas in all that I have observed of V. thapsus the anthers are oblique - These plants vary in appearance some have much more wooly leaves <>: others have leaves like V. L.’ (Cambridge University Library, 3r [CD 2]"
In a letter to his friend and Director of Kew, Joseph Hooker he wrote:
"Oct 6th Here is a fact which may possibly interest you. In a field here I find many Verbascum thapsus & lychnitis; & lots of varieties making an almost perfect series between those two distinct forms. I am sure many species have been run together on less perfect evidence. But lo & behold every one of these intermediate forms are absolutely sterile! & no doubt are natural hybrids. I found 33 of these hybrids in one field!!!"
Darwin wrote about hybridisation and sterility of Verbascum in The Variation of animals and plants under domestication (1868), see www.darwin-online.org.uk. In 1866, Darwin recorded that viper’s bugloss in the Cudham Valley appeared to have two different flower types.
"June 29th 1866. George gathered 15 flower-stalks of the Echium [viper’s bugloss] from field beyond orchis bank–not very dry–season has been wet. of these only 4 had long stamen & 11 had short imperfect stamens (Cambridge University Library, DAR 109: A15)."
These findings together with the results of previous investigations of this plant were reported in 'The different forms of flowers on plants of the same species' (1877) which can be read at www.darwin-online.org.uk
At the southern-most part of the Cudham Valley are Blackbush and Twenty Acre Shaws. These belong to the Woodland Trust and consists of semi-natural ancient woodland, secondary woodland, and chalk grassland. The woodbanks and pollarded trees of Blackbush and Twenty Acre Shaws, ancient in Darwin’s time, were part of the scenery with which he was familiar and support many species studied or observed by Darwin in woodland, grassland and hedgerow. Of these, Gentiana amarella (Autumn gentian) is uncommon elsewhere within the site. Others include Bryonia dioica (White bryony) Tamus communis (Black bryony) and 6 species of orchids.