Edward Forbes was at work on his celebrated paper in the "Geological Survey Memoirs" for 1846.

We have not seen the letter of Darwin's to which this is a reply, nor, indeed, any of his letters to Forbes.

The date of the letter is fixed by Forbes's lecture given at the Royal Institution on February 27th, 1846 (according to L.Horner's privately printed "Memoirs," II., page 94.)) Wednesday.

3, Southwark Street, Hyde Park.

[1846].
Dear Darwin To answer your very welcome letter, so far from being a waste of time, is a gain, for it obliges me to make myself clear and understood on matters which I have evidently put forward imperfectly and with obscurity.