From J. P. Thomasson   23 March 1875

Dear Sir,

May I venture to suggest that the following passage in the “Descent of Man &c” 1871, though modified in a later edition by the addition of the words “or conceal their nests”, still conveys a wrong impression: Part II. Page 170. “The two sexes of the common flycatcher (Muscicapa grisola) can hardly be distinguished, whilst the sexes of the pied flycatcher (M. luctuosa) differ considerably, & both build in holes”. I have found many nests of both species & my experience is, as far as I can remember, & after reading over my notes on both, that with Musc. luctuosa (atricapilla), the English pied flycatcher, the nest is well protected or concealed, whereas with Musc. grisola it is not concealed at all, or very imperfectly.¹

M^r. John Gibson quoted by Hewitson in his “Eggs of British Birds” 3^rd. Edition says that the hole chosen (by Musc. atricapilla) is generally too small to admit the hand,² which quite agrees with my experience, but I do not remember an instance where I could not place the hand in the nest of Musc. grisola, which is often built upon a branch of a pear tree against a garden wall, in a hole where a brick has fallen out, in ivy or in any small hollow against the trunk of a tree &c. I have also found it on the top of a tree root under a river bank, and against the side of a stump in an open space. I do not doubt however that the nest is sometimes inaccessible to the hand, as the bird will build almost in any hole or corner, but I have not found it in small holes—

I may perhaps mention that the eggs of these nearly allied species, contrary to the usual rule, differ much in colour, those of Musc. grisola being marked with red on a light ground, while those of Musc. atricapilla are of a light greenish blue. M^r. Hewitson states however that he has seen eggs of Musc. grisola in the collections of M^r. Wilmot & M^r. Salmon of a clear spotless blue—³ Musc. grisola is a widely distributed species, Musc. atricapilla much more local in its habitat, though abundant in some districts.

Do not trouble to reply to this letter.

With respect to consanguineous marriages I may mention, though isolated facts are probably of little value, that my wife’s Brother was born deaf & dumb, and her parents were second cousins.

He has two children, none of whom are affected—, nor are the three children of my wife—⁴

M^r. Samuel Smith, Chaplain to the Royal Association in aid of the Deaf & Dumb has a strong opinion that “the marriage of first cousins is undoubtedly the most prolific cause of congenital deaf-mutism known”, and in a late letter to the Times gives several lamentable instances in support of it.⁵

yours very truly | John P. Thomasson

Charles Darwin Esq

P.S. You may probably find the nest of Musc. grisola in your own garden early in June.

Posted 29 Mar. 75