To Asa Gray 11 August 1
(Down, Bromley Kent)
My dear Gray
Your note of July 27th has just reached me in Isle of Wight.—2 It is a real & great pleasure to me to write to you about my notions; & even if it were not so I shd be a most ungrateful dog, after all the invaluable assistance which you have rendered me, if I did not do anything which you asked.
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. Socy 3 I cannot give you facts, & I must write dogmatically, though I do not feel so on any point. I may just mention in order that you may believe that I have some foundation for my views, that Hooker has read my M.S, & though he at first demurred to my main point, he has since told me that further reflexion & new facts have made him a convert.— In the older or perhaps newer pliocene age (a little before Glacial epoch) the temperature was higher; of this there can be little doubt; the land on a large scale held much its present disposition: the species were mainly, judging from shells, what they are now.4 At this period, when all animals & plants ranged 10o or 15o nearer the poles, I believe the northern part of Siberia & of N. America being almost continuous were peopled (it is quite possible considering shallow water that Behrings straits were united, perhaps a little southward) by a nearly uniform Fauna & Flora, just as the Arctic regions now are.— The climate then became gradually colder till it became what it now is; & then the temperate parts of Europe & America would be separated, as far as migration is concerned, just as they now are.— Then came on the Glacial period, driving far south all living things; middle or even southern Europe being peopled with Arctic productions. as the warmth returned, the arctic productions slowly crawled up the mountains, as they became denuded of snow; & we now see on their summits the remnants of a once continuous flora & fauna.— This is E. Forbes’ theory, which however I may add I had written out 4 years before he published.5
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.6
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 shd beyond anything like to know whether you have any Alpine collections from Japan & what is their character.7
This letter is miserably expressed, but perhaps it will suffice to show what I believe has been the later main migrations & changes of temperature.—8
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.—9 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 shd draw in regard to Fu-mariaceæ.—
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.—10
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
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.
Please cite as
Darwin Correspondence Project, “Letter no. 2321,” accessed on 21 January 2017, http://www.darwinproject.ac.uk/DCP-LETT-2321