Darwin and Hooker on Hybrid Sterility

Comment:

Cross sterility refers to the infertility of a primary cross between two organisms.
Hybrid sterility refers to the infertility of the offspring of a successful primary cross. Thus, the primary cross would be successful, but the secondary cross would fail.
Dimorphic species have anatomically distinguishable male and female forms.

In June 1861, having completed a series of reciprocal crossing experiments on dimorphic species of Primula, Darwin wrote the first in an important series of dated theoretical notes in which he began to revise his former views on the question of the causes of cross and hybrid sterility. Darwin wrote notes on this subject intermittently throughout 1862, the last in the series being dated 4th January 1863. The notes are preserved in his portfolio of notes on hybridism (DAR 205.7:155-65).

Since Darwin subsequently reverted to his earlier [i.e. pre 1861] view on cross and hybrid sterility, only briefly referring to the ideas mentioned in these notes in his published work (see Origin 4^th ed, pp. 310-14, and Variation 2: 185-8), and since his notes shed light on the letters in this volume [see below], they have been reproduced here.

In Origin, Darwin dedicated an entire chapter to "Hybridism", noting at the outset that the theory of natural selection accorded a particular importance to the subject of hybrid sterility. He began (p. 245):

"The view generally entertained by naturalists is that species, when intercrossed, have been specially endowed with the quality of sterility, in order to prevent the confusion of all organic forms. . . . On the theory of natural selection the case is especially important, inasmuch as the sterility of hybrids could not possibly be of an advantage to them, and therefore could not have been acquired by the continued preservation of successive profitable degrees of sterility. I hope, however, to be able to show that sterility is not a specially acquired or endowed quality, but is incidental on other acquired differences."

Darwin's sense of the importance of the subject was increased by the response of Thomas Henry Thomas H. Huxley (1825-1895) Huxley to the theory of natural selection. From January 1860, Huxley repeatedly stated his view, first privately, then publicly, that natural selection could never be considered a vera causa for the origin of species until artificial selection had been shown to be capable of producing varieties of a species that were cross sterile (T. H. Huxley 1860a and 1860b). Darwin took this criticism seriously (see Correspondence vol. 8, letter to T. H. Huxley, 11th January [1860]), but attacked Huxley's tendency to imply that sterility was a "universal and infallible criterion of species" (see Correspondence vol. 8, letter to Charles Lyell, 12th [February 1860]).

To demonstrate that this was not the case, he repeatedly referred to the crossing experiments of Karl Friedrich von Gartner, already discussed in Origin, in which similarly coloured varieties of the same or different species of Verbascum were found to be less cross sterile than differently coloured varieties. Darwin considered the experiments so significant in countering Huxley's objection that he sought to replicate them himself (see Correspondence vol. 9, letter to J. D. Hooker, 28 September [1861]), telling Joseph Dalton Hooker:

"I do not think any experiment can be more important on Origin of species; for if he [Gartner] is correct, we certainly have what Huxley calls new physiological species arising."

However, Darwin could not obtain the requisite plants, and ultimately had to entrust the task to John Scott (see letter to W. E. Darwin 4th [July1862] and letter to John Scott, 11th December [1862] and n.19).

The subject of hybrid sterility continued to be the source of serious disagreement between Huxley and Darwin in 1862. When, in January, Huxley reiterated his views in his lectures at the Philosophical Institution of Edinburgh, telling Darwin that he had taken his "old line about the infertility difficulty" (see letter from T. H. Huxley, 13th January 1862), Darwin once again referred him to Gartner's experiments, but added: "Do oblige me by reading latter half of my Primula paper in Lin. journal, for it leads me to suspect that sterility will hereafter have to be largely viewed as an acquired or selected character. -- a view which I wish I had had facts to maintain in the Origin.--". (see letter to T. H. Huxley, 14th [January 1862]).

Darwin began to suspect that cross and hybrid sterility might be the result of natural selection following his experiments on dimorphic species of Primula Primula veris in the spring of 1861. In these experiments, Darwin carried out all four possible crosses between the two forms of flower in each species, namely two crosses between like forms (which he called "homomorphic", or, later, "illegitimate" crosses), and two between different forms (which he called "heteromorphic", or "legitimate" crosses). He discovered that the former crosses were much less fertile than the latter, and concluded that dimorphism occurred in order to favour cross-pollination, a finding in accord with his current work on the pollination of orchids, and with his longstanding conviction that an occasional cross was a "law of nature".

As the first of the dated notes reproduced below indicates, Darwin immediately began to see in these results implications for the question of hybrid sterility. In September 1861 he wrote to report his findings in Primula to Asa Gray, and to ask for details of any analogous cases of dimorphism, telling him: "This subject interests me much, so do help me if you can; for I have some very faint hopes that it may throw some light on Hybridisation" (Correspondence vol. 9, letter to Asa Gray, 16th September [1861]. When, shortly afterwards, he presented his results in a paper for the Linnean Society of London ("Dimorphic condition in Primula", p. 94; Collected papers 2: 61), he was more explicit):

"The simple fact of two individuals of the same undoubted species, when homomorphically united, being as sterile as are many distinct species when crossed, will surprise those who look at sterility as a special endowment to keep created species distinct. Hybridizers have shown that individual plants of the same species vary in their sexual powers, so far that one individual will unite more readily than another individual of the same species with a distinct species. Seeing that we thus have a groundwork of variability in sexual power, and seeing that sterility of a peculiar kind has been acquired by the species of Primula to favour intercrossing, those who believe in the slow modification of specific forms will naturally ask themselves whether sterility may not have been slowly acquired for a distinct object, namely, to prevent two forms, whilst being fitted for distinct lines of life, becoming blended by marriage, and thus less well adapted for their new habits of life".

The implications of this view of hybrid sterility were far-ranging, not only in addressing Huxley's objection to natural selection, but also (as Darwin's notes make clear) in addressing the issues of blending inheritance, and the role of geographic isolation in speciation. However, it was chiefly in regard to Huxley's objection that Darwin began to introduce his ideas to a number of correspondents in 1862. Having broached the subject with Huxley in January, he did not raise it directly for some months, merely hinting at the nature of his findings (see, for example, the letter to J. D. Hooker, 7 March [1862]). Meanwhile, he pursued an extensive research programme on dimorphism, repeating his earlier experiments on Primula veris, extending his experiments on P. sinensis to a second generation, experimenting on several species of Linum, and carrying out less detailed experiments on other dimorphic species. As a result, he became still more impressed by the degree of intraspecific sterility that could be obtained in dimorphic plants, telling Asa Gray that each of the two forms of Linum grandiflorum was so sterile with itself that, "taking sexual power as the criterion of difference, the two forms of this one species may be said to be generically distinct" (see letter to Asa Gray, 14 July [1862]). Soon afterwards, while occupied with a new enthusiasm for the trimorphic plant Lythrum salicaria, Darwin wrote to Gray with regard to his work on heterostyly: "to those who already believe in change of species, these facts will modify to [a] certain extent [the] whole view of Hybridity --" (see letter to Asa Gray, 9th August [1862]).

In December 1862, as Darwin reviewed his experiments on heterostyly, and discussed blending inheritance and the causes of variation with Hooker (see letters to J. D. Hooker, 24th [November 1862], [after 26] November [1862], and 12th [December 1862], and letters from J. D. Hooker, [15th and] 20th November [1862] and 26th November 1862), he once more ventured to disclose his private views on the origin of cross and hybrid sterility. In a letter dated 12th [December 1862], he told Hooker:

"By the way my notions on hybridity are becoming considerably altered by my dimorphic work: I am now strongly inclined to believe that sterility is at first a selected quality to keep incipient species distinct. If you have looked at Lythrum, you will see how pollen can be modified merely to favour crossing; with equal readiness it could be modified to prevent crossing.-- It is this which makes me so much interested with dimorphism etc."

Lythrum salicaria (Purple loosestrife)

Shortly afterwards, Darwin received another unpleasant surprise in the form of the published version of one of Huxley's recently delivered lectures to working men (T. H. Huxley 1862c; see letter to T. H. Huxley, 18th December [1862]). In the lecture, Huxley reiterated his views concerning natural selection, suggesting that the weakest point in Darwin's hypothesis was the lack of positive evidence that any groups of animals known to have been produced "by selective breeding from a common stock", had become mutually sterile (T. H. Huxley 1862c, pp. 108-13; see also pp.146-50). Darwin was exasperated by Huxley's criticisms, and especially by his claim that "the answer to varieties when crossed being at all sterile" was "absolutely a negative", which Darwin considered to be plainly contradicted by Gartner's hybridising experiments. "My God", he continued, "is not the case difficult enough without its being, as I must think, falsely made more difficult?" After a further exchange of letters (now missing), Darwin told his friend in a letter of 28th December [1862]: "We differ so much that it is no use arguing. To get the degree of sterility you expect in recently formed varieties seems to me hopeless."

Darwin's changing views about the selective origin of cross and hybrid sterility, together with Huxley's repeated criticisms, prompted him to seek further experimental evidence on the question, especially in regard to animals. Some time in December, Darwin sent out letters to a number of animal breeders inquiring about the occurrence of intra-specific sterility, although none of this correspondence has been found (see letters to W. B. Tegetmeier, 27th [December 1862] and 28th [December 1862]). In particular, he apparently suggested a breeding experiment to pigeon and fowl fanciers, designed to test whether hybrid sterility could be artificially selected among individuals of a single species. If successful, the experiment would, Darwin believed, "solve the problem of Sterility from Hybridism" (see letter to W. B. Tegetmeier, 27th [December 1862]). It would also have fully met Huxley's objection to the acceptance of natural selection as a vera causa. Indeed, Huxley had suggested something similar in his letter to Darwin of 20th January 1862, stating:

"I have told my students that I entertain no doubt that twenty years experiments on pigeons conducted by a skilled physiologist instead of a mere breeder-would give us physiological species sterile inter se from a common stock -- (and in this, if I mistake not, I go further than you do yourself) and I have told them that when these experiments have been performed I shall consider your views to have a complete physical basis -- and to stand on as firm ground as any physiological theory whatever."

Darwin discussed the proposed experiment with Huxley in his letter of 28th December [1862], telling him of his hope that the pigeon fancier and naturalist William Bernhard Tegetmeier would carry it out. At the start of December, Tegetmeier had obtained a grant of ten pounds from the Royal Society for "experiments on the crossbreeding of pigeons" (Royal Society, Council minutes, 1st December 1862). He originally planned the experiments to test "whether any existing breeds happen to have acquired accidentally any degree of sterility", a point that Darwin had previously tested, but Darwin encouraged him to carry out the experiments according to his own plan of selecting for sterility (see letter to W. B. Tegetmeier, 27th [December 1862]). In the event, however, Tegetmeier's experiments produced no sterile offspring (Variation 1:192).

In these letters of December 1862, Darwin began for the first time to commit himself more publicly to the idea that hybrid sterility was a selected quantity. However, the series of notes reproduced here ends abruptly with a note dated 4^th January 1863, in which Darwin twice rejects the selective origin of sterility: "It will not do". Moreover, between 1st April and 16^th June 1863, Darwin wrote a draft of the section of Variation dealing with "Crossing and Sterility": in the published version (Variation, chapters 25-9), he reverted to his earlier views on cross and hybrid sterility. Although Darwin revised his manuscript before it was finally published in January 1868, there is no evidence to suggest that the first draft differed from the published version in this regard.

In Variation 2: 185-9, Darwin referred to his former interest in the idea of the selective origin of cross and hybrid sterility, emphasising not only the evidence from heterostyled plants, but also the evidence that sterility had been acquired through natural selection for "other and widely different purposes, as with neuter insects in reference to their social economy" (p. 185). However, he cited three objections to this view:

Firstly, he noted that in cases of speciation where the incipient species were geographically isolated, cross and hybrid sterility had evidently been obtained without selection, since under such circumstances there could be no selective advantage attached to sterility.
Secondly, he pointed out that selection could not account for those instances where, in reciprocal crosses, "the male element of one form should have been rendered utterly impotent on a second form, whilst at the same time the male element of this second form is enabled freely to fertilise the first form" (p. 186), since selection ought to act equally on each of the reciprocal crosses.
Lastly, and most importantly, he argued that a "great difficulty" lay in "the existence of many graduated steps from slightly lessened fertility to absolute sterility".

He continued:

"It may be admitted, on the principle above explained, that it would profit an incipient species if it were rendered in some slight degree sterile when crossed with its parent-form or with some other variety; for thus fewer bastardised and deteriorated offspring would be produced to commingle their blood with the new species in process of formation. But he who will take the trouble to reflect on the steps by which this first degree of sterility could be increased through natural selection to that higher degree which is common to so many species, and which is universal with species which have been differentiated to a generic or family rank, will find the subject extraordinarily complex. After mature reflection it seems to me that this could have been of no direct advantage to an individual animal to breed badly with another individual of a different variety, and thus leave few offspring; consequently such individuals could not have been preserved or selected".

While Darwin considered that the case might be "somewhat different" with plants (p. 187), he pointed out that since cross and hybrid sterility followed "the same general laws in the vegetable as in the animal kingdom, it is improbable, though apparently possible, that, with plants, crossed species should have been rendered sterile by a different process" (p. 188). His conclusion was that cross and hybrid sterility had arisen "incidentally ... in connection with other and unknown changes in their organization".

Darwin's notes from 1861-3 include discussions of these potential objections, although it is clear that the third was uppermost in his mind during this period. The "mature reflection" which Darwin mentions in Variation clearly refers to the internal dialogue recorded in these notes.

[DAR 205-7: 155]

June 6th 1861. The case of Primula just opposite to Species; [In the case of species] similar forms are here fully fertile [whereas] dissimilar [forms are] sterile; with Primula the [anatomically] similar are sterile and the [anatomically] dissimilar fertile.

Keep in Hybrid Chapt[er.]

As we see sterility acquired merely to prevent intermarriage, and this can hardly be disputed by those who consider all [the] above facts and who believe in change of species; so may we not ask whether sterility may not have been acquired to prevent blending of two incipiently divergent forms. As one [organism] became adapted for aquatic life would it not have been an advantage that it sh[ould] not have become blended with the truly terrestrial form[?] This first change may have been infinitely slight, the degree [of] which my facts on Primula perhaps explain Gartner's ill-success in crossing Primrose and Cowslip. Did not Lecoq succeed. I have succeeded with greatest ease.

If we take [a] broad view of [the] good of Divergence- good of sterility follows.

In neuter insects sterility from loss of sexual instinct.

[DAR 205.7: 156]

Hybridism

Take [the] case of Pigeons paired for life -- with all [of] equal fixed fertility -- hardest case. Assume that [the] country will support a given number. But that with some sensible modification of structure and habit, a new species might be found[ed], and be supported in additional numbers. Each variety leading in this direction w[oul]d be preserved; but crossing with [the] old form would constantly drag back the incipient form. Now suppose a bird, varying slightly in [the] desired direction was born, which when paired with its nearest ally, was fully fertile, but was sterile with the old form; then its offspring which chanced to pair with its like would constantly be propagated, but those which propagated with the old would die out. -- we sh[oul]d have fewer of intermediate form, and more of the pure variety.

Take [the] case of a pigeon paired with similar var[iety of pidgeon], producing offspring, fully fertile with [their] own var[iety] and not fully fertile (in either way) with distinct var[iety; not their own]. Would this new little family A have [an] advantage in [the] long run over other similar var[ietie]s, which had no such tendency to sterility B? Many of As would die out from sterility; a few would pair and leave pure [members of their] own var[ietie]s and these would slowly increase. Of B all the gradation would live, yet occasionally some purely matched var[ietie]s would be produced. Occasionally perhaps [Darwin's emphasis] all Bs would match [cross] with other var[ietie]s and then [the] new variety would be lost. So it would be with A!!! Perhaps matched pairs if sterile would separate. My greatest difficulty [is] with animals. Why mere sexual predeliction does not by itself suffice; perhaps passion too strong. If no matching for life [the] case w[oul]d be clear: prepotency clear with plants.

[DAR 205.7: 157-8]

Sept. 21st 1861. (After considering Primula.)

The creationist who sees organisms from arctic and tropical countries, or from 2 continents, which never could meet, sterile together, must admit that incidental from endowed sterility to prevent forms which do come into contact from blending. So with sterility of Hybrid progeny.

Hence on view of sterility acquired by nat[ural] Selection no difficulty from such facts.

In animals [it is] hard to see why sterility sh[oul]d have been acquired. Why not mere instinctive dislike, which does prevail, why is sterility superadded [?] If [an] animal [is] born with slightly different habits of life and at same time by chance some sterility concurred, it would be [a] great advantage.

In plants, where [there is] no instinctive choice [of mate], such sterility [is] necessary.

Perhaps cases of Birds which pair for life [are the] greatest difficulty. How c[oul]d nat[ural] selection come in? Yes it could if new var[iety] paired with old it would be lost: if paired with a similar var[iety] it would be preserved. Sterility must supervene at [the] first formation of [the] Var[iety]. When we see how almost universal some sterility is, can we believe [it is] always acquired? Must be incidental. When whole body of a species changes like our Race-Horse, no use in [need for] sterility, --if one body changed in England and another in America, such 2 new forms would have no tendency to sterility if acquired through nat[ural] selection.

Gartner Bastard s. 165 on individuals refusing to cross. {seeing} Primula and Linum and neuter Hymenoptera and Neuroptera no difficulty in sterility being acquired, if this view would explain facts. It would, I think, apply to all offsets of one species in [the] same country. The Verbascum and Tobacco case shows incidental in some cases. (All the facts about Lobelia -- Passiflora etc., and Crinum crossing more readily with another species (this cannot be dimorphism) shows how easily [the] reproductive system is affected by [the] condition of life and rendered self sterile.). (Such cases as Rawson's Gladiolus must be [a] different case, i.e. good of crossing with distant individual.

I suspect it will come to this, that generally when a new form is selected within the same country with its parent, from early start sterility must be selected. The vast diversity in degree of sterility is compatible with its ensuing from various causes, -- correlation [with other selectable characters], as well as [direct] selection. Cases are on record of women sterile with one man and not with another.

Nov. 18^th 1861. We must look at selection for own (say) equation from steadily going on; but always retarded till sterility is at same time gained. I suspect this sterility must be habitually acquired before instinctive dislike acquired; otherwise this instinctive dislike would have been potent by itself and sterility would have been superfluous. Instinctive dislike perhaps does come much into play -- see multiple origin of domestic animals. Those that pair for life seems greatest difficulty.

With respect to {slow} modifications like our Race-Horses. Tendency to sterility may be inherited from former process of selection.

[DAR 205.7: 159-60]

Feb. 26^th 1862. It is true, as I said in Origin, no good to [a] creature to be sterile; but I forgot that this was only [the] means to [an] end. To keep var[ietie]s apart [is the] highest good, as we see by man, by fences, etc.. In plants it is prepotency of pollen which is probably aimed at. Though the sterility of parents and hybrids would be of good.

In animals my difficulty is that instincts would have sufficed; but one never knows how nature will work. Certainly instincts are not invariably strong enough to keep species apart. Hooded {illegible} Black {fans} Woodpecker etc., etc..

One can clearly see that under general point of view [for the general interest, it would be] good to keep var[ietie]s apart. If the hybrids could breed they would drag back the new var[ieties] to [the] primordial state.

Hence variation [is] so slow, as structural and physiological changes have to go together.

In all cases when animals pair and live in [a] herd under one Lord [dominant male], prepotency of semen (if it ever comes in) cannot apply.

Nature acts as a man would. If two of his best breeds united he would castrate the mongrel. As nature does in sterile [sterility] -- or neuter Hymenoptera and Neuroptera -- Border flowers of Compositae Umbelliferae -- Hydrangeae are rendered sterile: they perhaps as these cases due to laws of growth.

An pairing animal could not be selected, even if it varied in [the] right way, until it chanced to {exercise} either instinctive (and that would not occur early enough) or physiological aversion to unite with parent form. Perhaps [this would be] greatest first a[nd] little varieties and sub-var[ietie]s [would be] formed. This would be lost, unless it were separated by sterility from parent or other races.

[DAR 205.7: 161]

Nov. 18^th 1862. The fact proved by such a mass of evidence that crossing two individuals distinct, and probably longer kept distinct the better, adds to vigour and fertility of offspring, shows that there must be some excessively slight difference in pollen of every individual of [the] same species. Under unnatural conditions we find that the pollen gets so wonderfully altered (though effectual in itself) that, as in Lobelia, Potato, Passiflora, Crinum etc., the flower is quite sterile with its own pollen and requires that of another. This throws [a] flood of light on dimorphism and makes it not improbable (seeing the individual difference of pollen) that sterility may be selected to keep [the] var[ietie]s distinct.

[DAR 205.7: 162]

Dec. 3rd 1862. The structure of animals and plants, many decisive experiments, and common experience, shows that good is derived from crossing one individual with another of a distinct family or race. Now if, for instance, pollen from a distinct plant produces more numerous and more vigorous seedlings, than [a] flower's own pollen on the same stigma, then [the] two pollens must certainly be in some slight degree differentiated. Hence some differentiation of pollen is commonest Phenomenon. So must the stigma or ovules, for if pollen on 1st plant will be effective on the 2nd plant. The differentiation of female organ in [the] same plant is carried to extremes in certain plants under culture, as [in the cases of] Lobelia, Crinum, etc.. We thus see that dimorphism may readily arise, and we can understand why [its occurrence is] so variable within [members of the] same genus; how easily [dimorphism can be] lost under culture as in P. Sinensis, and in structure in P. auricula and Siberica (see J. Scott list). But if we once admit, the {easy} of differentiation of pollen, we can understand hybridity. P. sinensis shows how this sterility affects offspring. Natural selection only collects and forms this sterility. No doubt it is always in some degree correlated with changed structure.

[DAR 205.7: 163]

Perhaps important- Dec. 18^th. 1862.

{With} formation of marsh var[iety] of a Plant, it w[oul]d not be necessary that it sh[oul]d be sterile with [the] land var[iety], only that it produced rather more seed with [the] marsh var[iety], and the result would be that the marsh var[iety], having slight advantage of habit and greater fertility, would gain the end. This tendency which is, in fact, all that occurs in more fertile hybrids or {illegible} then might be selected or go on aggregating by simple effects of conditions.

Suppose a short-beaked var[iety] and [a] short-beaked var[iety], when paired produced on a verge a few more percentage of young ones than a heterogeneous pair -- this would clearly be [an] advantage -- (like seed in Wheat field) and so onwards. Those which produced most young when matched would necessarily prevail, subject to other contingencies.

A closely interbred dog, I have heard several cases, w[oul]d not breed with its own kind, but did freely with [a] mongrel. This serves to show how easily [the] generative system [can be] modified. Possibly or not some foal-getter may be perfectly fertile with certain individual females, but not with all.

[DAR 205.7: 164]

Jan 4th 1863. An animal, say, becoming adapted for aquatic life and surrounded by so many of its parent terrestrial forms, that in [the] course of [a] few generations all such var[ient]s chanced to get crossed by [with] them; [then it's line] would be dragged back from its favourable variation. But if a variation arose, which was sterile with [the] ordinary form, then even [if] so large a proportion united with the ordinary form, and were ultimately lost by sterility; yet if even so few united with [the] similar form, these would be kept pure; but then chances are supposed that they would unite with [the] ordinary form and so would be lost by sterility. It will not do. The sterility must supervene from being kept close to uniform condition of life for ages. If sexual disinclination supervened all would go well; but then why does sterility supervene, and why in [the] Hybrid [i.e. the second cross] and not always in [the] first cross. Will not do. [Darwin's emphasis]

N.B. Sexual disinclination must be an [Darwin's emphasis] acquired instinct; for how else could it arise?

[DAR 205.7: 165]

Make pollen of one var[iety] of Cabbage prepotent over another, or one var[iety] of Verbascum slightly prepotent over another. It may be, in some cases, incidental.. But .Whenever a new species has arisen, there has not always been another (as in such [a] case as [the] change of our Race-horse from [the] Arab[-horse]); this is a difficulty. So in [the] case of 2 species developed in 2 distant quarters of [the] world. Again, how are Hybrids themselves rendered sterile: this must be incidental. For I will never believe that Nat[ural] Selection was so blundering as to make the offspring sterile, and not the parents; moreover the relation of [the] sterility of two parents and [their] Hybrid child [is] not strictly correlated. N.B. we have sterility of another kind induced in neuter insects of several kinds. We could, on this view, understand [the fact that there is] no sterility in var[ietie]s selected by man. Perhaps understand the extreme variability in degree of sterility of Species -- Gartner's Verbascum case, and Maize, and Koh1reuter's tobacco, must be correlation [with some other character on which Natural Selection acts]. The case of Lobelia, Crinum and Passiflora, shows that under culture, the potentiality of pollen and stigma varies. It may be correlation in some cases and [direct] Selection in others: or increased by selection; but then the extreme and increasing degree of sterility as shown by different genera never crossing must be wholly due to correlation.

Mem. Reciprocal Union how could these be explained? What in case of Linum; may there not be [a] case with one form being dimorphic, and not another?

Hooker on Variation

From The Correspondence of Charles Darwin

Volume 10 for the year 1862, pp. 127-130.

Edited by F. Burkhardt, D. M. Porter, J. Harvey, & J. R. Topham (1997)

(With copyright permission from Cambridge University Press)
(Comments by DRF are noted as such, or are included in square brackets)

Kew Gardens circa 1790

Kew Gardens in about 1790

My dear Mr. Bates,

I have been thinking much of your extremely interesting letter before answering it, and shall be only too glad if I can say any thing that can tend to remove any difficulties. I need hardly commence by telling you that my opinions are nothing, that the whole question wants working out by observation, as you are doing, and by experiments which no one has attempted, or even suggested, that I know of. --- What I have said, and shall now say, you must take, not as opinions of mine, but as my tendencies of thought. -- Certainly, I incline to believe that Variation is sufficient to ensure any amount of divergence and that it (the principle or fact called Variation) is independent wholly [Hooker's emphasis] as to amount and kind of local circumstances.

My reasons for thus thinking are:

1) That it has never been shown experimentally that induced habits of the individual [which might cause variation] are propagated.
2) That no such effects of local circumstances on the individual [which might cause variation] is necessary.-- there being variation enough without it.
3) It seems more philosophical to suppose that the principle of Variation is one thing, immutable, and that local circumstances are the secondary causes acting through Natural selection on the varieties, killing some, sparing some, directing intermarriages, etc., etc..
4) That it would simplify matters very much if we could thus disentangle the two phenomena of Variation and Natural Selection.

I grant that not one of these reasons has a leg to stand upon in a strictly scientific point of view -- they are a string of hypotheses, and further, there are some seemingly opposed facts [apparent environmental influences]:

i) Hereditary diseases might have been originally induced by local causes; though I think they may be explained without it. It is said that a peculiar form of teeth becomes hereditary in syphilitic families.-- but I suspect that this could not be perpetuated -- it could only be propagated by intermarriage of children of syphilitic parents, which would surely die out.

ii) Then too I have heard that Dr Brown-Sequard has induced epilepsy in rabbits, and this has happened in the brood .-- if really so this is a strong point; but here again any attempt to perpetuate the brood of epileptics must end in extinction. All these are cases of induced diseases of the individual being propagated; -- could induced innocuous peculiarities be so propagated?
iii) Englishmen show no tendency to beardless faces after four generations of shaving -- and a thousand similar instances may be quoted.
iv) The oldest custom of all -- circumcision has had no effect on the organ of the race, after hundreds of generations: which I have always regarded as the most wonderful fact.
v) Again , though the habit of inducing varieties in plants and animals has gone on from time immemorial, no one has ever supposed that one place is better than another [in] causing the first brood to vary. -- Whoever plants most gets most varieties, i.e .secures most new sorts by after selection, etc., etc., etc..
vi) To put the question in its simplest form -- no one ever supposed that of 12 peas in one pod, 6 grown in England would be more different inter se, or from their parents, than the other 6 grown in Australia. Again, as I have remarked in the Introd: Essay to New Zealand, the trueness with which seeds from all parts of the world come up in our gardens is astounding. -- Whoever heard of a new species thus raised by the first sowing? -- Were it otherwise -- did individual seeds come up differently in different localities -- you would surely, in extreme cases, have different species and different genera coming up at once in our gardens. -- On the other hand, Rivett wheat seed produces Rivetts in Australia the first year; but whereas even Rivett wheat has its varieties etc., and some of them are more suited to Australia than England, after several generations the Rivetts of Australia will differ from those of England, but through Natural Selection. In all these considerations we must carefully exclude mere stunting, or the effect of overgrowth and undergrowth, which are not variations in the sense I allude to, though I confess to my inability of pointing out any scientific distinction that is irrefragable.

Now all your apparent exceptions are, I grant, very strong cases at first sight; but may all be explained by assuming more time than you do, and correcting more than you do for the millions of lost individuals. That "species will be constant under one set of conditions and variable under another" is quite true, but this is no proof that even extreme external physical conditions have acted on the pregnant female so as to have produced greater differences amongst her ova, -- or on the sperm cells of the male previously. -- to an extent perceptible in the first brood.

I have long, on other grounds, denied that tropical heat or light produces the bright coloring of plants, or that arctic climates produce woolly covering. These phenomena are far too partial to be attributed to such cosmical phenomena. You say "you are convinced that inorganic conditions have some effect". I am quite ready to believe it when I see any one experimental case, or such an accumulation of arguments pro, as I think can be adduced con. It is an open question, I grant, quite so, but you must bring your case to the point. You say "you cannot see that the species has produced the same variations in all the stations" etc.. How should you ever see whether or not, except [unless] you compare [the] progress of individuals of [the] same brood in each [case]? Again, you ask "if variety A succeed in locality I, why should it not succeed at locality II if it had ever been existent there?" My answer is that no locality II is identical with a locality I and that Natural Selection will act on an imperceptible difference. It searches [Hooker's emphasis] where no faculty of man can follow.

Consider again and ponder well the number of individuals of a brood that die for every one that lives, multiply these by the countless generations that it must have taken to have established that amount of change which we call specific, and then reflect that if, besides all this, we are to have direct [Hooker's emphasis] effects of local conditions, which (remember) vary in kind and amount from year to year, and you will have such an accumulation of change-effecting forces that there could be no such things as recognisable species and genera. Not only every place would have different varieties, but every year in each place, and in the case of temperate plants grown in a hothouse, wet country plants in a dry house, etc., etc., etc., we should have startling changes, ever occurring.

Another objection to my line of argument is the changes wrought in bees by either feeding or heat (as the case may be). But this again is change of [the] individual, and is not propagated, for the Queen after all lays again males and drones, not Queens. Darwin I believe holds with you as to the influence of external conditions on the variation of the brood. I have, however, failed to be convinced by him of it, and I do not think he recognizes the facts of variation to the extent I do. Indeed I think his book would have been more convincing had he treated Variation somehow so as to have impressed the unaccustomed reader and thinker to regard it as the origin of species and Natural Selection as the fixer of these. As it is, in most minds the two are confused, or Natural Selection is supposed to make the varieties as well as to fix them. At other times no one more staunchly denounces the effects of external circumstances in producing Variation than Darwin does.

Darwin also believes in some reversion to type which is opposed to my view of Variation. You may have a single character persist or reappear, but the sum of differences goes on increasing as you depart from the parent. Variation I hold to be centrifugal; if it were not so, how could it go on making species, which are only the preserved forms of each brood which circumstances favoured? Remove the circumstances which kill the others, as man does when he cultivates, (who kills those that nature would have spared), and you have what you call a variety, and fancy [that] you made it, whereas you only prevented nature killing it.

After all, Darwin's axiom that man has never failed in getting varieties of any species he has fairly tried, is in favor of my view that the abstract principle called Variation is enough with time, to beget any amount of change; and by means of Natural Selection to retain only such as may present any amount of difference. Finally I have come to look upon the Law of Variation as I do on gravitation: local circumstances may mask its effects, but upon itself they cannot act. The power that drives the stone to the ground is the same whether the stone permeates water or air or vacuum, -- whether you let it fall straight, or throw it forwards, or even upwards. So, a certain amount of centrifugal force of variation is distributed in certain proportions amongst the 12 peas in the pod; and except to arrest or retard the progress and amount of development of the individuals or their organs (I incline to think) local circumstances are powerless. Give more Nitrogen to Pea Number 1 and you will have more and greener leaves; but its seeds, again, will not be as green as [if] they too are not supplied with nitrogen.

I think my long letter will disgust you with asking any more questions; but I should be greatly obliged if you have time and would write me again on the subject. I know no one but yourself who is really thinking out Darwin's views to any purpose in Zoology. I am sure that with you, as with me, the more you think, the less occasion you will see for anything but time and Natural Selection to effect change, and that this view is the simplest and clearest in the present state of science, is one advantage at any rate. Indeed, I think that it is, in the present state of the enquiry, the legitimate position to take up; it is time enough to bother our heads with a secondary cause when there is some evidence of it, or some demand for it. At present I do not see one or other, and so I feel inclined to renounce any other for the present. It is not so very long since I thought differently and that Variation was the first effect of circumstances on the individual.

We are sorry to hear of your Influenza. I hope it is gone now. We are all quite well, and the children returned to school. I hope when you next come to town you will let me know, and that we may have some more boxes of butterflies at the Linnean, and get the many curious facts you name well impressed in the languid circulation of the Entomologist.

Ever most truly yours, Jos. D Hooker

Letter from Joseph D. Hooker, at the Royal Gardens, Kew, to Charles Darwin

November 26^th 1862

Joseph D. Hooker (1817-1911)

Dear Darwin,

I return Asa Gray's letter with a thousand thanks. I am very glad to see it -- sorry too -- how odd it is that men in his position have not learnt by experience that they are no judges of contemporaneous events.

He has made a great blunder in his criticism on Oliver, who was aghast (poor man) till I pointed it out. Breeding-in [inbreeding] does not favor Variation, as he supposes. He mistakes the "perpetuation of a variety", for the "propagation of variation", which is a totally different thing. Close breeding [in-breeding] will, of course, tend to preserve and multiply a variety in as much as every individual is a variety (he forgets that the type is a myth), whereas crossing tends to [produce] Variation by adding differences to preexisting ones. I have several times caught A. Gray in lacking precision of thought, --logic in short...[Section omitted by DRF] .

By the way, when you have any difficulties, such as believing too much in [the] action of physical conditions, you must do as the parsons tell their flocks, come to me, [Hooker's emphasis] or some other wise and discreet… etc., etc., etc.. You may as well talk to me of expressing the Glory of the Almighty as of ditto to Natural Selection. I am a jolly good neophyte. I do however calmly think that there is still amongst us some confusion of ideas between "action of Physical causes" and "Effects of Physical causes".

I am still very strong in holding to impotence of crossing with respect to [the] origin of species. I regard Variation as so illimitable in {animals}. You must remember that it is neither crossing nor Natural Selection that has made so many divergent human individuals, but simply Variation [Hooker's emphasis]. Natural Selection, no doubt, has hastened the process, intensified it (so to speak), has regulated the lines, places, etc., etc., etc., in which, and to which, the races have run and led, and the number of each, and so forth; but, given a pair of individuals with power to propagate, and [an] infinite [time] span to procreate in, so that not one be lost, or that, in short, Natural Selection is not called on to play a part at all, and I maintain that after n generations you will have extreme individuals as totally unlike one another as if Natural Selection had extinguished half.

If once you hold that Natural Selection can make a difference, i.e. create a character, your whole doctrine tumbles to the ground. Natural Selection is as powerless as physical causes to make a variation; the law that "like shall not produce like" is at the bottom of [it] all, and is as inscrutable as life itself. This it is that Lyell and I feel you have failed to convey with force enough to us and the public: and this is at the bottom of half the infidelity of the scientific world to your doctrine. You have not, as you ought, begun by attacking old false doctrines, that "like does produce like". The first chapter of your book should have been devoted to this and to nothing else.

Comment by DRF:

Heredity and Variation are facts. Heredity means that "like" tends to produce "like". Variation means that Heredity is not perfect, so that "like" does not produce an exact copy of the original form. Thus, Hooker's Law of Variation is that "like shall not produce like".

At issue, is whether Variation is intrinsic (like gravity), or occurs because of some aspect of "external conditions". After Variation has occurred, Natural Selection can be the "fixer" of the variations ("survival of the fittest").

But Hooker also points out that fixation can occur in the absence of Natural Selection ("survival of the survivors"). This is best equated with what we now call "Random Drift".

But there is some truth I now see in the objection to you, that you make Natural Selection the "Deus ex machina", for you do somehow seem to do it, by neglecting to dwell on the facts of incessant variation. Your 8 children are really all totally [Hooker's emphasis] unlike one another; they agree exactly [Hooker's emphasis] in no one property. How is this? You answer that they display the inherited differences of different progenitors -- well -- but go back, and back and back in time, and you are driven at last to your original pair for origin of differences; and logically you must grant, that the differences between the original male and female of your species were equal to the sum of the extreme differences between the most dissimilar existing individuals of your species! -or that the latter varied from some inherent law that had them. Now am not I a cool fish to lecture you so glibly?

No steps will be taken regarding Owen and I am glad of it. Let his wickedness find him out of itself. I shall read Falconer's paper with great interest. I shall not send Oxalis this weather-- without you wish it. It is of no value, but would disappoint you, I fear, these sensitive things want fine warm sunny weather.

It is no bore to write to you God knows, it is jolly good fun and what a relief from Welwitschia! I shall look at Lythrum tomorrow (your drawing I mean), and [the] description, a thousand thanks for it, I wanted much to know it.

Huxley has just sent me his Number 1. working mens' lectures. The one only wise, good, and conservative thing I ever did, was to hold out against lecturing for love or money or fame -- it is equally admirable -- whether you call it a horribly selfish act -- or a 'cute sense of my own inability --, or [a] piece of confounded lazyness -- to all which motives I plead proudly guilty.

And I am your dear friend. J. D. Hooker.

PS. Another Box of Welwitschia has arrived at Lisbon en route for me. I am fainting away.

My dear Hooker,

Your last letter has interested me to an extraordinary degree, and your truly parsonic advice "some other wise and discreet person" etc., amused us not a little. I will put a "concrete" case to show what I think A. Gray believes about crossing and what I believe. Rock Doves from Diane Jacky's web pages If 1000 pigeons were bred together in [a] cage for ten 1000 years, their number not being allowed to increase (by chance killing), then from [because of] mutual intercrossing no varieties would arise; but if each pigeon were a self-fertilising hermaphrodite a multitude of varieties would arise. This I believe is [the] common effect of crossing, viz. the obliteration of incipient varieties. I do not deny that when two marked varieties have been produced; their crossing will produce a third, or more, intermediate varieties. Possibly, or probably with domestic varieties, with [a] strong tendency to vary, the act of crossing tends to give rise to new characters; and thus a third or more races, not strictly intermediate, may be produced. But there is heavy evidence against new characters arising from crossing wild forms; only intermediate races are then produced. Now do you agree thus far? If not, it is no use arguing, we must come to swearing, and I am convinced I can swear harder than you. Therefore. I am right. Q. E. D.

If the number of 1000 pigeons were prevented increasing, not by chance killing, but by, say, all the shorter-beaked birds being killed, then the whole [Darwin's emphasis] body would come to have longer beaks. Do you agree?

Thirdly, if 1000 pigeons were kept in [a] hot country, and another 1000 in [a] cold country, and fed on different food, and confined in [a] different size aviary, and kept constant in number (by chance killing), then I should expect, as rather probable, that after ten 1000 years, the two bodies would differ slightly in size, colour, and perhaps other trifling characters. This I should call the direct action of physical conditions. By this action I wish to imply that the innate vital forces are somehow led to act rather differently in the two cases. Just as heat will allow or cause two elements to combine, which otherwise would not have combined. I should be especially obliged if you would tell me what you think on this head.

But the part of your letter which fairly pitched me head over heels with astonishment is that where you state that every single difference which we see might have occurred without any selection. I do and have always fully agreed; but you have got right round the subject and viewed it from an entirely opposite and new side, and when you took me there, I was astounded. When I say I agree, I must make proviso, that under your view, as now, each form long remains adapted to certain fixed conditions, and that the conditions of life are in [the] long run changeable; and second, which is more important, that each individual form is a self-fertilising hermaphrodite, so that each hairbreadth variation is not lost by intercrossing. Your manner of putting [the] case would be more striking than it is, if the mind could grapple with such numbers -- it is grappling with eternity; -- think of each of a thousand seeds bringing forth its plant, and then each a thousand. A globe stretching to [the] furthest fixed star would very soon be covered. I cannot even grapple with [the] idea even with races of dogs, cattle, pigeons or fowls; and here all must admit and see the accurate strictness of your illustration.

Such men, as you and Lyell thinking that I make too much of a Deus of Natural Selection is conclusive against me. Yet I hardly know how I could have put in, in all parts of my Book, stronger sentences. The title, as you once pointed out, might have been better. No one ever objects to agriculturalists using the strongest language about their selection; yet every breeder knows that he does not produce the modification which he selects. My enormous difficulty for years was to understand adaptation, and this made me, I cannot but think rightly, insist so much on Natural Selection. God forgive me for writing at such length; but you cannot tell how much your letter has interested me. And how important it is for me with my present Book in hand to try and get clear ideas. Do think a bit about what is meant by direct action of physical conditions. I do not mean whether they act; my facts will throw some light on this. I am collecting all cases of "bud-variations in contradistinction to "seed-variation"" (do you like this term for what some gardeners call "sports"?): these eliminate all effect of crossing. Pray remember how much I value your opinion, as the clearest and most original I ever get.

Very sincere thanks to you and Oliver for the Books: Planchon has been very useful: they shall be all returned by Rail the first day, probably Tuesday, on which I send to Bromley.

I see plainly Wellwitschia will be a case of Barnacles.

Please do not send Oxalls sensitiva.

I have another plant to beg, but I write on separate paper, as more convenient for you to keep. I meant to have said before, as excuse for asking for so much from Kew; that I have now lost two seasons, by accursed nurserymen not having right plants, and sending me the wrong instead of saying that they did not possess.

Ever yours. My dear Hooker. C. Darwin

My good old friend,

How kind you have been to give me so much of your time! Your letter is of real use and has been, and shall be, well considered. I am much pleased to find that we do not differ as much as I feared. I begin my book with saying that my chief object is to show [the] inordinate scale of variation; and I have especially studied all sorts of little variations of the individual.

On cross[ing] I cannot change; the more I think, the more reason I have to believe that my conclusion would be agreed to by all practised breeders. I, also, greatly doubt about variability and domestication being at all necessarily correlative; but I have touched on this in Origin. Plants being identical under very different conditions has always seemed to me a very heavy argument against what I call direct action.

I think perhaps I will take [the] case of 1000 pigeons as [a] means to sum up my volume. I will not discuss other points; but as I have said I shall recur to your letter. But I must just say that if sterility be allowed to come into play -- if long-beaked be in least degree sterile with short-beak, my whole case is altered. By the way, my notions on hybridity are becoming considerably altered by my dimorphic work: I am now strongly inclined to believe that sterility is at first a selected quality to keep incipient species distinct. If you have looked at Lythrum you will see how pollen can be modified merely to favour crossing; with equal readiness it could be modified to prevent crossing. It is this which makes me so much interested with dimorphism, etc..

One word more: when you pitched me head over heels by your new way of looking at the back side of Variation, I received assurance and strength by considering monsters,-- due to law, but so horribly strange as they are. I looked at some Plates; the monsters were alive till at least when born. They differ at least as much from parent as any one mammal from another.

I have just finished a long weary chapter on simple facts of variation of cultivated Plants; and am now refreshing myself with [a] paper on Linum for [the] Linnean Society... [Section omitted by DRF]

My dear Hooker. Yours ever most truly.

C. Darwin.

My dear Sir,

I thank you sincerely for your letter, and am heartily glad to hear of R.S. making so good a move. I am, however, not sanguine of success. The present plan is to try whether any existing breeds happen to have acquired accidentally any degree of sterility; but to this point hereafter. The enclosed M.S. [manuscript] will show what I have done and know on the subject. Please at some future time carefully return the MS. to me. If I were going to try again, I would prefer [?crossing] Turbit with Carrier or Dragon.

I will suggest an analogous experiment, which I have had for two years in my Experimental book with "be sure & try", but which, as my health gets yearly weaker and weaker and my other work increases, I suppose I shall never try. Permit me to add that if 5 pounds would cover [the] expences of [the] experiment, I should be delighted to give it and you could publish result if there be any result. I crossed Spanish Cock (your bird) and white Silk hen, and got plenty of eggs and chickens; but two of these seemed to be quite sterile. White silkie bantam

I was then sadly overdone with work but have ever since much reproached myself, that I did not preserve and carefully test the procreative power of these hens. Now, if you are inclined to get a Spanish Cock and a couple of white Silk hens, I shall be most grateful to hear whether the offspring breed well; they will prove, I think, not hardy; if they should prove sterile, which I can hardly believe, they will anyhow do for the [cooking] pot.

conchin hens If you do try this; how would it be to put a silk cock to your curious silky Cochin Hen; so as to get a big Silk breed; it would be curious if you could get silky fowl with bright colours. I believe a silk hen crossed by any other breed never give silky feathers. A cross from Silk Cock and Cochin Silk Hen ought to give silky feathers and probably bright colours.

I have been led lately from experiments (not published) on Dimorphism to reflect much on sterility from Hybridism and partially to change the opinion given in Origin. I have now letters out enquiring on [the] following point, implied in the experiment, which seems to me well-worth trying, but too laborious ever to be attempted. I would ask every Pigeon and Fowl Fancier, whether they have ever observed in the same breed, a cock A paired to a hen B, which did not produce young. Then I would get cock A and match it to a hen of its nearest blood; and hen B [match] to its nearest blood. I would then match the offspring of A (viz: A, B, C, D, E) to the offspring of B (viz: F, G, H, I, J), and all these children which were fertile together should be destroyed until I found, one, (say A) which was not quite fertile with (say I) Then A and I should be preserved and paired with their parents A and B, so as to try and get two families, which would not unite together; but the members within each family being fertile together. This would probably be quite hopeless; but he who could effect this, would, I believe, solve the problem of Sterility from Hybridism.

If you should ever hear of individual fowls or pigeons which are sterile together, I should be very grateful to hear of [the] case. It is parallel case to those recorded of a man not impotent long living with a woman who remained childless; the husband died and the woman married again and had plenty of children. Apparently (by no means certainly) this first man and woman were dissimilar in their sexual organisation. I conceive it possible that their offspring (if both had married again and both had children) would be sexually dissimilar like their parents, or sterile together.

Pray forgive my dreadful writing; I have been very unwell all day, and have no strength to rewrite this scrawl. I am working slowly on, and I suppose in 3 or 4 months shall be ready for M.S. of Fowls.

My dear Sir. Yours very sincerely. C. Darwin

I am sure I do not know whether any human being could understand or read this shameful scrawl.