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Darwin Correspondence Project

From Asa Gray   30 June 1855

Harvard University, Cambridge, U.S.

June 30th, 1855

My dear Mr. Darwin,

Your long letter of the 8th inst. is full of interest to me, and I shall follow out your hints as far as I can. I rejoice in furnishing facts to others to work up in their bearing on general questions, and feel it the more my duty to do so in as much as, from preoccupation of mind & time & want of experience, I am unable to contribute direct original investigations of the sort to the advancement of science

Your request at the close of your letter, which you have such needless hesitation in making, is just the sort of one which it easy for me to reply to, as it lies directly in my way. It would probably pass out of my mind, however, at the time you propose,—so I will attend to it at once, to fill up the intervals of time left me while attending to one or two pupils.— So I take some unbound sheets of a copy of the Manual, and mark off the “close species’,1 by connecting them with a bracket

Those thus connected some of them I should in revision unite under one— —many more Dr. Hooker would unite toute suite, and for the rest it would not be extraordinary if, in any case, the discovery of intermediate forms compelled their union.

As I have noted on the blank pages of the sheets I send you (thro’ Sir Wm. Hooker) I suppose that if we extended the area—say to that of our Flora of North America2 —we should find that the proportion of “close species” to the whole flora increased considerably. But here I speak at a venture. Some day I will test it for a few families.

If you take for comparison with what I send you, the British Flora,3 or Koch’s Fl. Germanica4 —or Godron’s Flora of France5 —& mark the ‘close species” on the same principle, you will doubtless find a much greater number.— Of course you will not infer from this that the two floras differ in this respect; since the difference is probably owing to the fact that, 1.—there have not been so many observers here bent upon detecting differences, & 2. Our species—thanks mostly to Dr. Torrey & myself—have been more thoroughly castigated. What stands as one species in the Manual would figure in almost any European Flora as 2, 3, or more in a very considerable number of cases.

In boldly reducing nominal species Joe Hooker is doing a good work; but his vocation—like that of any other reformer—exposes him to temptations and dangers.

Because you have shown that a and b. are so connected by intermediate forms that we cannot do otherwise than regard them as variations of one species, we may not conclude that c & d. differing much in the same way and to the same degree, are of one species before an equal amount of evidence is actually obtained. That is, when two sets of individuals exhibit any grave differences, the burden of proof of their common origin lies with the person who takes that view: and each case must be dicided on its own evidence, and not on analogy, if our conclusions in this way are to be of real value. Of course we most often jump at conclusions from imperfect evidence.

I should like to write an essay on species, some day; but before I should have time to do it, in my plodding way, I hope you, or Hooker, will do it, and much better far. I am most glad to be in conference with Hooker & yourself, on these matters, and I think we may, or rather you may, in a few years settle the question as to whether Agassiz’s—or Hooker’s views are correct: they are certainly widely different.6

Apropos to this: many thanks for the paper containing your experiments on seeds exposed to sea-water. Why has nobody thought of trying the experiment before! instead of taking it for granted that salt water kills seeds— I shall have it nearly all reprinted in Silliman’s Journal,7 as a nut for Agassiz to crack.8

Dr. Alexander asks me to enquire of Miss Morris about a story of hers of seeing a beetle with eggs of fish attached. He says he has written to get information about it for you, but no reply comes.9 This Miss Morris is a good observer. I know her sister very well, & shall write to-day, & hope to get the statement of the facts for you, very soon.

Ever, My dear Sir | Yours faithfully | Asa Gray

P.S. I had overlooked your P.S. & must send one in reply.

I wrote hap-hazard of the distances between our alpine summits; but will now endeavor to ascertain more correctly. I was probably thinking of the distance by the roads, rather than in a direct air line.

I was wide enough of the mark I see— From the Alpine summits of White Mts. of New Hampshire to Mansfield Mt. or other alpine tops of Green Mountains is only about 75 miles direct, by my maps.— I suppose English Statute miles. From Camel’s Rump, Green Mts. to Mt. Marcy, New York about 60 miles. I should have looked at the map before writing. Much obliged to you for recalling my attention to it. | A. G.

CD annotations

0.3 My Dear … pupils.— 2.5] crossed pencil
3.4 a few families. 3.5] ‘It shd be remembered if wide area be taken the standard of comparison wd. also be increased.’ added pencil
6.6 and each case … evidence. 6.9] double scored pencil
8.1 Apropos … Gray 10.1] crossed pencil
11.1 P.S… . line. 12.3] crossed pencil
Top of first page: ‘(On close Species)’pencil
Bottom of first page: ‘6’ pencil


The list of ‘close species’ was actually sent as a four-page manuscript, not, as Gray says, on proof-sheets of A. Gray 1848. See letter to J. D. Hooker, 14 August [1855]. The list is annotated by CD and is in DAR 165: 92/3.
Gray’s Manual (A. Gray 1848) treated the botany of the northern United States. Torrey and Gray 1841–3 dealt with the entire area of North America.
William Jackson Hooker’s British flora, first published in one volume in 1830, ran to many editions. Gray probably refers to the expanded edition of 1850, in which W. J. Hooker was joined by George Arnott Walker Arnott (W. J. Hooker and Arnott 1850). CD’s copy of the seventh edition (W. J. Hooker and Arnott 1855) is in the Darwin Library–CUL.
Grenier and Godron 1848–55.
At the time, both Joseph Dalton Hooker and Louis Agassiz regarded species to be fixed, but Hooker thought all species originated from a single parent or pair whereas Agassiz believed in multiple creation.
Gray reprinted CD’s letter to the Gardeners’ Chronicle, 21 May [1855], in American Journal of Science and Arts 2d ser. 20 (1855): 282–4 (issued in September 1855). Gray added the comment: ‘We have just learned from Mr. Darwin that some of these seeds have germinated after 82 and 85 days’ immersion in sea-water, namely, those of Radishes, Beet, Atriplex, Capsicum, Oats, Cucurbita, Rhubarb, Lettuce, Carrot, Celery, and Onions’ (p. 284).
Much of Agassiz’s argument for the simultaneous creation of many individuals throughout the geographic area they were destined to inhabit was based on the belief that migration and colonisation were too irregular and inadequate to explain the existing distribution patterns of animals and plants over the earth.
The information on Dyticus marginalis (a misspelling of Dytiscus marginalis,the great diving beetle) had already been dispatched. See letter from M. H. Morris to R. C. Alexander, 17 June 1855.


Gray, Asa. 1848. A manual of the botany of the northern United States, from New England to Wisconsin and south to Ohio and Pennsylvania inclusive. Boston and Cambridge: James Monroe and Company. London: John Chapman.

Gray, George Robert. 1841. A list of the genera of birds, with their synonyma and an indication of the typical species of each genus. 2d edition. London: Richard and John E. Taylor.

Koch, Wilhelm Daniel Joseph. 1843–4. Synopsis florae Germanicae et Helveticae, exhibens stirpes phanerogamas rite cognitas, praemissa generum dispositione secundum classes et ordines systematis Linnaeani conscripta. 2d edition. 2 vols. Frankfurt: Fridericus Wilmans. Leipzig: Gebhardt & Reisland.


Sends a list of "close" species from his Manual of botany.

Hopes Hooker or CD will write an essay on species. Discusses some of the difficulties of defining botanical species.

Letter details

Letter no.
Asa Gray
Charles Robert Darwin
Sent from
Harvard University
Source of text
DAR 165: 92a
Physical description
ALS 5pp †

Please cite as

Darwin Correspondence Project, “Letter no. 1707,” accessed on 23 May 2024,

Also published in The Correspondence of Charles Darwin, vol. 5