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* direct »Letter 383 — Darwin, C. R. to Geological Society, 20 Oct 1837
Report on J. G. Forchhammer's communications on changes of levels of land in Denmark [Proc. Geol. Soc. Lond. 2 (1838): 554–6].
* direct »Letter 534 — Humboldt, F. W. H. A. von to Darwin, C. R., 18 Sept 1839
Praises CD's Journal of researches and comments on some of CD's observations and conclusions. Considers volcanic activity and its effect on past climate and changes in climate over time. Discusses glacial phenomena. Believes the climate of the coast of Peru is modified by cold sea-currents.
* direct »Letter 573 — Darwin, C. R. to Henslow, J. S., 3 July [1840]
Describes an orange tree with curious "horned" fruit; sends specimen. Asks if the horns represent "metamorphoses of some organ into the fruit orange".
* direct »Letter 693 — Darwin, C. R. to Gardeners' Chronicle, [late Aug 1843]
Sends some examples of Gentiana that he thinks may shed light on the origin of double flowers. Since specimens grew in sterile soil their double flowering cannot be attributed to excess food. CD advances the hypothesis that some change in natural conditions causes sterility, which then causes compensatory development of petals, the organs closest in morphology to those whose functions have been checked.
* direct »Letter 776 — Darwin, C. R. to Hooker, J. D., [8 Sept 1844]
Acknowledges note and parcel for Ehrenberg. Considers why different areas have different numbers of species. Gives an example opposing JDH's view that paucity of species results from vicissitudes of climate. CD has concluded that species are most numerous in areas that have most often been divided, isolated from, and then reunited with, other areas. Cannot give detailed reasons but believes that "isolation is the chief concomitant or cause of the appearance of new forms".
* direct »Letter 964 — Hooker, J. D. to Darwin, C. R., [25 Mar 1846]
JDH recognises the existence of "altered states" of continental species in island floras. The botanists' difficulty in determining a new species is no grounds for dismissing the important question of altered forms. Will look for Ascension plants for Ehrenberg. French Galapagos collections confirm JDH's view that plants arrived from north. Cannot agree with Forbes on North Atlantic flora. Botanical definition of "highness" and "lowness" usually means complexity and simplicity. Some plants, such as aquatic ones, are cleistogamous. Cannot see why they should not be.
* direct »Letter 1545 — Darwin, C. R. to Cresy, Edward, Jr, [before May 1848?]
Obliged for account of change in quality of wool. "Some authors will not admit that climate has any perceptible action." Hopes his health is re-established.
* direct »Letter 1175 — Darwin, C. R. to Herschel, J. F. W., 11 May [1848]
Describes colour changes from blue to red in valves of operculum of the cirripede genus Ibla.
* direct »Letter 1590 — Darwin, C. R. to Huxley, T. H., 8 Sept [1854]
Agrees with THH on metamorphosis of branchiae of Balanus, and on his view of Owen.
* direct »Letter 1817 — Blyth, Edward to Darwin, C. R., 8 Jan 1856
Encloses "notes for Mr. D" [see 1818] and a memorandum on the wild cattle of southern India [see 1819]. Breeds of silky fowl of China and Malaya. Black-skinned fowl. Doubts any breed of canary has siskin blood; all remain true to their type. Wild canary and finch hybrids.Hybrids between one- and two-humped camels. Does not regard zebra markings on asses as an indication of interbreeding but as one of the many instances of markings in the young which more or less disappear in the adult. Crossing of Coracias species at the edges of their ranges. Regional variations and intergrading between species of pigeons. Regards the differences in Treron as specific [see Natural selection, p. 115 n. 1]. Gives other instances of representative species or races differing only in certain details of colouring.