Life and Letters of Charles Darwin — Volume 2

The work on heterostyled plants has a special bearing, to which the author attached much importance, on the problem of origin of species. (See 'Autobiography,' volume i.)

He found that a wonderfully close parallelism exists between hybridisation and certain forms of fertilisation among heterostyled plants. So that it is hardly an exaggeration to say that the "illegitimately" reared seedlings are hybrids, although both their parents belong to identically the same species. In a letter to Professor Huxley, my father writes as if his researches on heterostyled plants tended to make him believe that sterility is a selected or acquired quality. But in his later publications, e.g. in the sixth edition of the 'Origin,' he adheres to the belief that sterility is an incidental rather than a selected quality. The result of his work on heterostyled plants is of importance as showing that sterility is no test of specific distinctness, and that it depends on differentiation of the sexual elements which is independent of any racial difference. I imagine that it was his instinctive love of making out a difficulty which to a great extent kept him at work so patiently on the heterostyled plants. But it was the fact that general conclusions of the above character could be drawn from his results which made him think his results worthy of publication. (See 'Forms of Flowers,' page 243.)

The papers which on this subject preceded and contributed to 'Forms of Flowers' were the following: —

"On the two Forms or Dimorphic Condition in the Species of Primula, and on their remarkable Sexual Relations." Linn. Soc. Journal, 1862.)

"On the Existence of Two Forms, and on their Reciprocal Sexual Relations, in several Species of the Genus Linum." Linn. Soc. Journal, 1863.

"On the Sexual Relations of the Three Forms of Lythrum salicaria," Ibid. 1864.

"On the Character and Hybrid-like Nature of the Offspring from the Illegitimate Unions of Dimorphic and Trimorphic Plants." Ibid. 1869.

"On the Specific Differences between Primula veris, Brit. Fl. (var. Officinalis, Linn.), P. vulgaris, Brit. Fl. (var. acaulis, Linn.) and P. elatior, Jacq.; and on the Hybrid Nature of the Common Oxlip. With Supplementary Remarks on Naturally Produced Hybrids in the Genus Verbascum." Ibid. 1869.

The following letter shows that he began the work on heterostyled plants with an erroneous view as to the meaning of the facts.]

CHARLES DARWIN TO J.D. HOOKER. Down, May 7 [1860].

... I have this morning been looking at my experimental cowslips, and I find some plants have all flowers with long stamens and short pistils, which I will call "male plants," others with short stamens and long pistils, which I will call "female plants." This I have somewhere seen noticed, I think by Henslow; but I find (after looking at my two sets of plants) that the stigmas of the male and female are of slightly different shape, and certainly different degree of roughness, and what has astonished me, the pollen of the so-called female plant, though very abundant, is more transparent, and each granule is exactly only 2/3 of the size of the pollen of the so-called male plant. Has this been observed? I cannot help suspecting [that] the cowslip is in fact dioecious, but it may turn out all a blunder, but anyhow I will mark with sticks the so-called male and female plants and watch their seeding. It would be a fine case of gradation between an hermaphrodite and unisexual condition. Likewise a sort of case of balancement of long and short pistils and stamens. Likewise perhaps throws light on oxlips...

I have now examined primroses and find exactly the same difference in the size of the pollen, correlated with the same difference in the length of the style and roughness of the stigmas.

CHARLES DARWIN TO ASA GRAY. June 8 [1860].

... I have been making some little trifling observations which have interested and perplexed me much. I find with primroses and cowslips, that about an equal number of plants are thus characterised.

SO-CALLED (by me) MALE plant. Pistil much shorter than stamens; stigma rather smooth, — POLLEN GRAINS LARGE, throat of corolla short.

SO-CALLED FEMALE plant. Pistil much longer than stamens, stigma rougher, POLLEN-GRAINS SMALLER, — throat of corolla long.

I have marked a lot of plants, and expected to find the so-called male plant barren; but judging from the feel of the capsules, this is not the case, and I am very much surprised at the difference in the size of the pollen... If it should prove that the so-called male plants produce less seed than the so-called females, what a beautiful case of gradation from hermaphrodite to unisexual condition it will be! If they produce about equal number of seed, how perplexing it will be.

CHARLES DARWIN TO J.D. HOOKER. Down, December 17 [1860?].

... I have just been ordering a photograph of myself for a friend; and have ordered one for you, and for heaven's sake oblige me, and burn that now hanging up in your room. — It makes me look atrociously wicked.

... In the spring I must get you to look for long pistils and short pistils in the rarer species of Primula and in some allied Genera. It holds with P. Sinensis. You remember all the fuss I made on this subject last spring; well, the other day at last I had time to weigh the seeds, and by Jove the plants of primroses and cowslip with short pistils and large grained pollen (Thus the plants which he imagined to be tending towards a male condition were more productive than the supposed females.) are rather more fertile than those with long pistils, and small-grained pollen. I find that they require the action of insects to set them, and I never will believe that these differences are without some meaning.

Some of my experiments lead me to suspect that the large-grained pollen suits the long pistils and the small-grained pollen suits the short pistils; but I am determined to see if I cannot make out the mystery next spring.

How does your book on plants brew in your mind? Have you begun it?..

Remember me most kindly to Oliver. He must be astonished at not having a string of questions, I fear he will get out of practice!

[The Primula-work was finished in the autumn of 1861, and on November 8th he wrote to Sir J.D. Hooker: —

"I have sent my paper on dimorphism in Primula to the Linn. Soc. I shall go up and read it whenever it comes on; I hope you may be able to attend, for I do not suppose many will care a penny for the subject."

With regard to the reading of the paper (on November 21st), he wrote to the same friend: —

"I by no means thought that I produced a "tremendous effect" in the Linn. Soc., but by Jove the Linn. Soc. produced a tremendous effect on me, for I could not get out of bed till late next evening, so that I just crawled home. I fear I must give up trying to read any paper or speak; it is a horrid bore, I can do nothing like other people."

To Dr. Gray he wrote, (December 1861): —

"You may rely on it, I will send you a copy of my Primula paper as soon as I can get one; but I believe it will not be printed till April 1st, and therefore after my Orchid Book. I care more for your and Hooker's opinion than for that of all the rest of the world, and for Lyell's on geological points. Bentham and Hooker thought well of my paper when read; but no one can judge of evidence by merely hearing a paper."

The work on Primula was the means of bringing my father in contact with the late Mr. John Scott, then working as a gardener in the Botanic Gardens at Edinburgh, — an employment which he seems to have chosen in order to gratify his passion for natural history. He wrote one or two excellent botanical papers, and ultimately obtained a post in India. (While in India he made some admirable observations on expression for my father.) He died in 1880.

A few phrases may be quoted from letters to Sir J.D. Hooker, showing my father's estimate of Scott: —

"If you know, do please tell me who is John Scott of the Botanical Gardens of Edinburgh; I have been corresponding largely with him; he is no common man."

"If he had leisure he would make a wonderful observer; to my judgment I have come across no one like him."

"He has interested me strangely, and I have formed a very high opinion of his intellect. I hope he will accept pecuniary assistance from me; but he has hitherto refused." (He ultimately succeeded in being allowed to pay for Mr. Scott's passage to India.)

"I know nothing of him excepting from his letters; these show remarkable talent, astonishing perseverance, much modesty, and what I admire, determined difference from me on many points."

So highly did he estimate Scott's abilities that he formed a plan (which however never went beyond an early stage of discussion) of employing him to work out certain problems connected with intercrossing.

The following letter refers to my father's investigations on Lythrum (He was led to this, his first case of trimorphism by Lecoq's 'Geographie Botanique,' and this must have consoled him for the trick this work played him in turning out to be so much larger than he expected. He wrote to Sir J.D. Hooker: "Here is a good joke: I saw an extract from Lecoq, 'Geograph. Bot.,' and ordered it and hoped that it was a good sized pamphlet, and nine thick volumes have arrived!"), a plant which reveals even a more wonderful condition of sexual complexity than that of Primula. For in Lythrum there are not merely two, but three castes, differing structurally and physiologically from each other:]

CHARLES DARWIN TO ASA GRAY. Down, August 9 [1862].

My dear Gray,

It is late at night, and I am going to write briefly, and of course to beg a favour.

The Mitchella very good, but pollen apparently equal-sized. I have just examined Hottonia, grand difference in pollen. Echium vulgare, a humbug, merely a case like Thymus. But I am almost stark staring mad over Lythrum (On another occasion he wrote (to Dr. Gray) with regard to Lythrum: "I must hold hard, otherwise I shall spend my life over dimorphism."); if I can prove what I fully believe, it is a grand case of TRIMORPHISM, with three different pollens and three stigmas; I have castrated and fertilised above ninety flowers, trying all the eighteen distinct crosses which are possible within the limits of this one species! I cannot explain, but I feel sure you would think it a grand case. I have been writing to Botanists to see if I can possibly get L. hyssopifolia, and it has just flashed on me that you might have Lythrum in North America, and I have looked to your Manual. For the love of heaven have a look at some of your species, and if you can get me seed, do; I want much to try species with few stamens, if they are dimorphic; Nesaea verticillata I should expect to be trimorphic. Seed! Seed! Seed! I should rather like seed of Mitchella. But oh, Lythrum!

Your utterly mad friend, C. DARWIN.

P.S. — There is reason in my madness, for I can see that to those who already believe in change of species, these facts will modify to a certain extent the whole view of Hybridity. (A letter to Dr. Gray (July, 1862) bears on this point: "A few days ago I made an observation which has surprised me more than it ought to do — it will have to be repeated several times, but I have scarcely a doubt of its accuracy. I stated in my Primula paper that the long-styled form of Linum grandiflorum was utterly sterile with its own pollen; I have lately been putting the pollen of the two forms on the stigma of the SAME flower; and it strikes me as truly wonderful, that the stigma distinguishes the pollen; and is penetrated by the tubes of the one and not by those of the other; nor are the tubes exserted. Or (which is the same thing) the stigma of the one form acts on and is acted on by pollen, which produces not the least effect on the stigma of the other form. Taking sexual power as the criterion of difference, the two forms of this one species may be said to be generically distinct.")

[On the same subject he wrote to Sir Joseph Hooker in August 1862: —

"Is Oliver at Kew? When I am established at Bournemouth I am completely mad to examine any fresh flowers of any Lythraceous plant, and I would write and ask him if any are in bloom."

Again he wrote to the same friend in October: —

"If you ask Oliver, I think he will tell you I have got a real odd case in Lythrum, it interests me extremely, and seems to me the strangest case of propagation recorded amongst plants or animals, viz. a necessary triple alliance between three hermaphrodites. I feel sure I can now prove the truth of the case from a multitude of crosses made this summer."

In an article, 'Dimorphism in the Genitalia of Plants' ('Silliman's Journal,' 1862, volume xxxiv. page 419), Dr. Gray pointed out that the structural difference between the two forms of Primula had already been defined in the 'Flora of North America,' as DIOECIO-DIMORPHISM. The use of this term called forth the following remarks from my father. The letter also alludes to a review of the 'Fertilisation of Orchids' in the same volume of 'Silliman's Journal.']

CHARLES DARWIN TO ASA GRAY. Down, November 26 [1862].

My dear Gray,