From James Croll   23 June 1869

Dear Sir,

Please to accept of my warmest thanks for the copy of the new edition of your Origin of Species which you were so kind as to present me with.¹ I trust that you will not consider me vain for saying that I am very much gratified with the complimentary way in which you have referred to my papers.² I am particularly interested in the section on alternate glacial periods in north and south.³ I had no idea of this application of the theory when engaged on the subject or I might have brought out the point more clearly than I have done, that when the northern hemisphere, for example, is under a glacial period the line of highest temperature will not be at the equator but will lie a very considerable distance to the south of the equator. And again when the glacial period is transferred over to the southern hemisphere the line of greatest heat will move over to as great a distance to the north of the equator. Your idea that the temperate climate plants will move up the mountain side while this line of greatest heat is being transferred from the one hemisphere to the other is most ingenious as it is natural.

I am, | Yours very truly | James Croll

Charles Darwin Esq. M.A; F.R.S.