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* direct »Letter 429a — Darwin, C. R. to Henslow, J. S., 3 November 1838
Relates plan for an appendix to his Journal of researches which will include facts of species of birds' being different in different islands of the Galápagos and also of the lizards and tortoises on the islands. Asks JSH whether he can supply parallels in the plant life.
* direct »Letter 457 — Yarrell, William to Darwin, C. R., [c. 17 Dec 1838]
Extract of a letter from Sir Robert Heron to WY, copied for CD, about the crossing of solid- and divided-hoofed pigs, and Angora rabbits of different colours.
* direct »Letter 502 — Darwin, C. R. to Herbert, William, [c. 1 Apr 1839]
Questions on breeding of plants: variation in established versus new varieties; predominance of wild species and old varieties when crossed with newer forms; predominance of males versus females; correlations between ease of hybridisation and tendency to vary and undergo cultivation; reversion; correlations between hybridisation and geographic distribution. In WH's Amaryllidaceae [1837], does he intend to say crossing is inimical to fertility? [Sent via J. S. Henslow; note to amanuensis Syms Covington.]
* direct »Letter 503 — Herbert, William to Henslow, J. S., 5 Apr 1839
Replies to CD's questions on plant hybridisation and laws of inheritance. Rejects predominant transmission of characters by established forms. Males show predominance, but congeniality of parents' constitution to climate and soil more important. No correlation between hybridisation and variability, cultivation, and geographical distribution. Rejects reversion. Describes experiments in Hippeastrum in which pollen from another species proved more fertile than plant's own pollen. Did not intend to say that crossing is inimical to fertility.
* direct »Letter 523 — Darwin, C. R. to Herbert, William, 26 June 1839
CD is led to believe there are no true permanently inbreeding, sexually reproducing beings. Thanks for replies to breeding questions. Asks for clarification of Hippeastrum crosses: is selfing or crossing with individual of same species intended and was increased fertility due to constitution of foreign parent or due to the pollen coming from another plant? Has WH known any hybrid or mongrel to revert or to vary in a manner unlikely to be effect of soil? Sends Journal of researches.
* direct »Letter 543 — Darwin, C. R. to Henslow, J. S., [10 Nov 1839]
Urges JSH to describe Galapagos species in a paper on the flora of the islands. Has been interested in geographical distribution and would be interested to have a paper by JSH on the general character of flora of Tierra del Fuego and Patagonia. "I keep on steadily collecting every sort of fact which may throw light on the origin & variation of species."
* direct »Letter 618 — Darwin, C. R. to De la Beche, H. T., 7 Feb 1842
Asks De la Beche about variation among domesticated animals in Jamaica.
* direct »Letter 789 — Darwin, C. R. to Hooker, J. D., [10 Nov 1844]
Origin of Antarctic brash ice. Further on case of Lycopodium: does JDH know any genera of plants whose species are variable in one continent but not in another? Discussion on variations between floras as regards species richness, and factors affecting geographical distribution. On species, CD expects "that I shall be able to show even to sound naturalists that there are two sides to the question of the immutability of species; – that facts can be viewed and grouped under the notion of allied species having descended from common stocks". Mentions books and papers for and against species mutability. CD believes past absurd ideas arose from no one's having approached subject on side of variation under domestication. Would like to see Clarke's paperand would welcome visit from JDH.
* direct »Letter 791 — Hooker, J. D. to Darwin, C. R., 14 Nov 1844
Differences in variability of species within a single genus. Further observations on Lycopodium. Interested in Humboldt's river with different floras on opposite banks, and other unexplained cases of very local distributions.
* direct »Letter 884 — Hooker, J. D. to Darwin, C. R., [mid-July 1845]
The translation of Humboldt's Kosmos [Cosmos (1846–58)] is delayed. Gives instances of peculiar genera with several good species in very small islands. Scarcity of insects on islands. JDH cannot prove that there is much hybridising, but does not see why there should not be. "Bother variation, development & all such subjects, it is reasoning in a circle I believe after all."