My dear Darwin,

Very very many thanks for the capital photograph of your excellent self;¹ I value it greatly; it recalls to my memory so vividly and pleasantly the delightful day I spent with you and our dear friends Hooker and Harvey at Nuneham many years ago.²

I am glad you were pleased with the flowers of the Cassia:³ my friend M^r. Glenie is so delighted at having been able to communicate something to interest you that he promises to observe carefully the changes that take place in the flowers of the different species of Cassia during the time they are expanded.⁴ With reference to another species of Cassia (C. Roxburghii, DC.)⁵ he writes me “We have been examining the C. Roxburghii of late carefully, and find in it, as well as in C. Fistula, a peculiarity though not of the same marked nature, in the anthers.— The 3 upper appear to be abortive;—the middle 4 open by pores only; the lower 3 (the longest) open by pores and by clefts at the base of the lobes”

I will collect for you and send you as soon as I can, flowers of all the Cassias and other Cæsalpinieæ I can get here. It seems likely to me that the careful study of the differences inter se of the closely allied forms or species of one group, and the comparison of these with corresponding differences in other groups, will be one of the modes of iluminating the law which has operated in bringing about these differences, whether it be that of natural selection or of a law operating with it or independently of it; for such a vast period of time seeming to be a necessary element in the process, there would appear to be slight hope of direct experiment (in selection) leading to any result beyond what we see in the races of dogs, or of other domestic animals producing fertile crosses. But do these fertile crosses between very dissimilar races shew any tendency to degenerate or disappear after a certain number of generations? The mixed race here, between the Portuguese and Cinghalese, are not a healthy set of people judging from those I have had anything to do with, and they certainly will not compare in symmetry or good looks with either of the original races.

Believe me always | My dear Darwin | Most sincerely yours | G. H. K. Thwaites.