Species migration since the Pliocene. Effect of the glacial epoch. Present geographical distribution, especially similarities of mountain floras, explained by such migration; mountain summits as remnants of a once continuous flora and fauna.
Cross-fertilisation in Fumariaceae.
(Down, Bromley Kent)
My dear Gray
Your note of July 27
I have discussed in long M.S. Ch. the later changes of climate &
effects on migration & I will here give you an abstract of
abstract, (which latter I am preparing of my whole work for Linn.
Some facts have made me vaguely suspect that between glacial & present temperature there was a period of slightly greater warmth.— According to my modification-doctrines, I look at many of the species of N. America which closely represent those of Europe, as having become modified since the pliocene period, when in northern part of world there was nearly free communication between old & new worlds.
But now comes a more important consideration; there is considerable body of geological evidence that during Glacial epoch the whole world was colder: I inferred this many years ago from erratic Boulder phenomena carefully observed by me on both E & W coasts of S. America. Now I am so bold as to believe that at the height of Glacial epoch, & when all Tropical productions must have been considerably distressed, that several temperate forms slowly travelled into the heart of the Tropics & even reached the southern hemisphere; & some few southern forms penetrated in reverse direction northward (Heights of Borneo with Australian forms) (Abyssinia with Cape Forms) Wherever there was nearly continuous high land, this migration would have been immensely facilitated: hence European character of plants of T. del Fuego & summit of Cordillera: hence ditto on Himalaya. As the temperature rose, all the temperate intruders would crawl up the mountains. Hence the European forms on Nilgherries, Ceylon, summit of Java—Organ Mountains of Brazil.— But these intruders being surrounded with new forms would be very liable to be improved or modified by natural selection to adapt them to the new forms with which they had to compete: hence most of the forms on the mountains of the Tropics are not identical but representative forms of N. temperate plants.—
There are similar classes of facts in marine productions. All this will appear very rash to you & rash it may be; but I am sure not so rash as it will at first appear to you; Hooker could not stomach it all at first, but has become largely a convert.
From Mammalia & shallow sea, I believe Japan to have been joined to mainland of
China within no remote period: & then the migration n. &
S. before, during & after glacial epoch would act on Japan, as on
corresponding latitude of China & U. States. I
This letter is miserably expressed, but perhaps it will suffice to show what I believe has been the later main migrations & changes of temperature.—
With respect to our dispute on Fumariaceæ, I am delighted to see that we now
absolutely agree; for I never supposed the structure of their flowers to do
more than favour an occasional cross, perhaps only once in several
generations.— But have you attended to one point:
plant a cabbage or radish of 2 distinct varieties moderately near each other,
& the proportion of mongrelised plants is immense: indeed sometimes hardly any
come true from seed raised under such circumstance. I have counted
proportions.— Now the stigma of each flower is sur-rounded not only by its own
6 stamens, with pollen shed as soon as flowersare open, but by a multitude of
other flowers with pollen of the same variety; & yet this mongrelising takes
place to an enormous extent. I believe a cross is so beneficial, that the pollen of a
distinct variety has a prepotent action over the plant's own pollen.
You will see the inference which I sh
Whenever my abstract is published next winter, I will of course send you a copy; though by the way I shall not give abstract on facts in regard to crossing, for they are too many to abstract.—
I fear that this letter & my horridly bad handwriting will be a trouble you to read.
Believe me | My dear Gray | Yours most sincerely | Ch. Darwin
- f1 2321.f1Dated by CD's reference to being on the Isle of Wight. He visited the island from 17 July to 13 August 1858 (‘Journal’; Appendix II).
- f2 2321.f2Gray's note has not been found. It was a reply to CD's letter to Asa Gray, 4 July 1858.
- f3 2321.f3See letter to J. D. Hooker, [5 August 1858] and n. 5.
- f4 2321.f4CD's explanation of the effect of climate on the geographical distribution of animals and plants is given in Origin, pp. 365–82. See also Natural selection, pp. 534–66.
- f5 2321.f5CD refers to the sketch of his species theory that he wrote in 1842 (Foundations, pp. 30–1), in which he anticipated the theory given by Edward Forbes in Forbes 1846. See Correspondence vol. 3.
- f6 2321.f6For Joseph Dalton Hooker's views on CD's theories of climatic change and geographical distribution, see Correspondence vols. 3, 5, and 6.
- f7 2321.f7Gray had published a description of a small collection of Japanese plants in 1857 and was preparing a second paper on the plants collected on the United States North Pacific Expedition (A. Gray 1859). See Correspondence vol. 6, letter from Asa Gray, 16 February 1857.
- f8 2321.f8For Gray's use of the information given in this letter, see letter to J. D. Hooker, 11 May .
- f9 2321.f9See letter from Asa Gray, 21 June 1858 and letter to Asa Gray, 4 July 1858.
- f10 2321.f10CD's material on crossing (Natural selection, pp. 35–91) was used in Variation.