Ɛpsilon

To Robert McLachlan   23 March [1866]1

Down Bromley Kent

Mar 23

Dear Sir

I am very much obliged to you for your paper on Sterrha, which I have read with very great interest.2 Many of your observations are quite new to me. I was particularly glad to read about the varying organs in the Neuroptera;3 should you ascertain similar cases in the Trichoptera,4 I shd be grateful for information.

Would it not be well worth while to get some of your friends to breed Eupithecia from eggs of the same female & to feed them on different plants & thus ascertain positively how the mimicry is effected.5

I have been very glad to see (whether or not you have been influenced by my writings) that you have given up to a great extent the belief in the immutability of species;6 & I feel sure that as you attend to other subjects besides the discrimination of species you will ultimately go further in your belief; at least I have hitherto found this to be the case with those who have doubted to a limited degree.

With my best thanks believe me dear Sir | yours very faithfully | Ch. Darwin

LS

Endorsement: ‘1866 | C. Darwin | Down | 23 March’

1
An incomplete version of the letter text taken from a sale catalogue transcription was published in Correspondence vol. 18, Supplement. This version is from a facsimile.
2
CD refers to McLachlan’s paper ‘On some remarkable varieties of Sterrha sacraria Linn., with general notes on variation in Lepidoptera’ (McLachlan 1865). Sterrha sacraria is now Rhodometra sacraria, the vestal. It belongs to the family Geometridae.
3
See McLachlan 1865, p. 467. The order Neuroptera is now restricted to lacewings and their kin, but used to include dragonflies and damselflies, which are now in the order Odonata (see also n. 6, below).
4
Trichoptera is the order of caddisflies.
5
Eupithecia is a genus of the family Geometridae; it includes most of the moths commonly known as pugs. McLachlan had observed that, among Lepidoptera, while the larva in this genus showed the greatest tendency to variation, the imago was most constant. He suggested that the larval variation was related to a tendency of the larva to mimic the colour of the food plant on which it developed (see McLachlan 1865, p. 465).
6
McLachlan had referred to transitional species of Gomphus (club-tailed dragonflies, then in the order Neuroptera) as evidence for development theory (McLachlan 1865, p. 467).

Please cite as “DCP-LETT-5038F,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-5038F