My dear Hooker

Your letter has pleased me much, for I never can get it out of my head, that I take unfair advantage of your kindness, as I receive all & give nothing. What a splendid discussion you could write on whole subject of variation! The cases discussed in your last note are valuable to me, (though odious & damnable) as showing how profoundly ignorant we are on causes of variation.—

I shall just allude to these cases, as a sort of sub-division of polymorphism—a little more definite I fancy than the variation of for instance the Rubi, & equally or more perplexing.—²

I have just been putting my notes together on variations apparently due to the immediate & direct action of external causes;³ & I have been struck with one result. The most firm stickers for independent creation admit, that the fur of same species is thinner towards south of range of same species than to north—that same shells are brighter coloured to S. than N.; that same is paler-coloured in deep water—that insects are smaller & darker on mountains—more lurid & testaceous near sea—that plants are smaller & more hairy & with brighter flowers on mountains: now in all such (& other cases) cases, distinct species in the two zones follow the same rule, which seems to me to be most simply explained by species, being only strongly marked varieties, & therefore following same laws as recognised & admitted varieties. I mention all this on account of variation of plants in ascending mountains; I have quoted the foregoing remark only generally with no examples, for I add there is so much doubt & dispute what to call varieties; but yet I have stumbled on so many casual remarks on varieties of plants on mountains being so characterised, that I presume there is some truth in it. What think you? do you believe there is any tendency in varieties, as generally so called, of plants to become more hairy & with proportionally larger & brighter coloured flowers in ascending a mountain.—

I have been interested in my “weed garden” of 3 × 2 feet square:⁴ I mark each seedling as it appears, & I am astonished at number that come up. & still more at number killed by slugs &c.— Already 59 have been so killed; I expected a good many, but I had fancied that this was a less potent check than it seems to be; & I attributed almost exclusively to mere choking the destruction of seedlings.— Grass-seedlings seem to suffer much less than exogens.—

I have almost finished my floating experiments on salt-water:⁵ $\frac{72}{94}$ sunk under 10 days—seven plants, however, floated on average 67 days each.— I then dried all these (with in each case, with pods, bits of twig & few leaves) & $\frac{62}{94}$ sunk under 10 days, so that generally the drying had no great effect, but here comes the odd part sometimes it had great effect, thus

Not dried Dryed

floated floated

Asparagus 22–23 85–86 (germinated excellently)

Lychnis dioica 21–22 44–45

Honeysuckle 3–4 21–22

Heloscadium 1–2 21–22 & seeds above 90 days & germinated splendidly

Not Dryed Dryed

Barbery 20–21 40–41

Viscaria oculata 2–3 30–31

Dianthus 1–2 28–29

Sweet Briar 2–3 21–22

Juniper 12–13 38–39

Nuts 0–1 60–70

&c &c &c &c

I think it will turn out on average from my very few experiments, of very little value, but better than mere conjecture, that about $\frac{1}{10}$1/10 of all plants of a country will float when dryed 30 days & the seeds then germinate; & this on average current of 33 miles per day will carry them a good way.⁶ I would wager that the pods of the Acacia(?) scandens which get to the Azores had been dried first.—⁷ I suppose the oriental species does not fruit at Kew: if it did, I sh^d. like to try.—

Are there any hardy garden plants or shrubs endemic to the Canaries, Madeira or Azores.— I sh^d. like to know; as it shows that the constitution of an endemic plant is not absolutely fitted to its home, perhaps in more striking manner than the hardiness or naturalisation of a plant from a continent.— The Chiococca racemosa, the suckers of which I cannot weed out of my garden, is a very striking case, as it is, I believe, confined to W. Indies.⁸

Farewell my dear Hooker, everything which I write about or think of, I long to talk over with you, as I have shown in this note.

Farewell | C. Darwin

P.S Strictly according to my experiments a little above $\frac{1}{7}$ (.140) of the plants of any country could be transported 924 miles & would then germinate! for $\frac{18}{94}$ have floated above 28 days & $\frac{64}{87}$64/87 is proportion of seeds which germinate after 28 days immersion.— & average of current in Atlantic is 33 miles per diem.—

I have just had a letter from Emma & she speaks with much pleasure at having seen you & M^rs. Hooker, whose state (you are as bad as I am) is to be pitied.⁹

P.S. Can any general character be predicated of water-plants; if so, & again if any plant has a variety growing in damp ground, does it take in ever so slight a degree the characteristic features of aquatic plants.— It w^d. be another case to the many which I have collected.

A plant abruptly having two forms like the aquatic Ranunculus seems something different & very unpleasant to me.