More Letters of Charles Darwin Volume II

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LETTER 580. TO W.J. HOOKER.

Testimonial from Charles Darwin, Esq., M.A., F.R.S. and G.S., late
Naturalist to Captain Fitz-Roy's Voyage.

Down House, Farnborough, August 25th, 1845.

I have heard with much interest that your son, Dr. Hooker, is a candidate
for the Botanical Chair at Edinburgh. From my former attendance at that
University, I am aware how important a post it is for the advancement of
science, and I am therefore the more anxious for your son's success, from
my firm belief that no one will fulfil its duties with greater zeal or
ability. Since his return from the famous Antarctic expedition, I have
had, as you are aware, much communication with him, with respect to the
collections brought home by myself, and on other scientific subjects; and I
cannot express too strongly my admiration at the accuracy of his varied
knowledge, and at his powers of generalisation. From Dr. Hooker's
disposition, no one, in my opinion, is more fitted to communicate to
beginners a strong taste for those pursuits to which he is himself so
ardently devoted. For the sake of the advancement of Botany in all its
branches, your son has my warmest wishes for his success.

LETTER 581. TO J.D. HOOKER.
Down, Thursday [June 11th, 1847].

Many thanks for your kindness about the lodgings--it will be of great use
to me. (581/1. The British Association met at Oxford in 1847.) Please
let me know the address if Mr. Jacobson succeeds, for I think I shall go on
the 22nd and write previously to my lodgings. I have since had a tempting
invitation from Daubeny to meet Henslow, etc., but upon the whole, I
believe, lodgings will answer best, for then I shall have a secure
solitary retreat to rest in.

I am extremely glad I sent the Laburnum (581/2. This refers to the
celebrated form known as Cytisus Adami, of which a full account is given in
"Variation of Animals and Plants, " Volume I., Edition II., page 413. It
has been supposed to be a seminal hybrid or graft-hybrid between C.
laburnum and C. purpureus. It is remarkable for bearing "on the same tree
tufts of dingy red, bright yellow, and purple flowers, borne on branches
having widely different leaves and manner of growth." In a paper by
Camuzet in the "Annales de la Societe d'Horticulture de Paris, XIII., 1833,
page 196, the author tries to show that Cytisus Adami is a seminal hybrid
between C. alpinus and C. laburnum. Fuchs ("Sitz. k. Akad. Wien," Bd. 107)
and Beijerinck ("K. Akad. Amsterdam," 1900) have spoken on Cytisus Adami,
but throw no light on the origin of the hybrid. See letters to Jenner Weir
in the present volume.): the raceme grew in centre of tree, and had a most
minute tuft of leaves, which presented no unusual appearance: there is now
on one raceme a terminal bilateral [i.e., half yellow, half purple] flower,
and on other raceme a single terminal pure yellow and one adjoining
bilateral flower. If you would like them I will send them; otherwise I
would keep them to see whether the bilateral flowers will seed, for Herbert
(581/3. Dean Herbert.) says the yellow ones will. Herbert is wrong in
thinking there are no somewhat analogous facts: I can tell you some, when
we meet. I know not whether botanists consider each petal and stamen an
individual; if so, there seems to me no especial difficulty in the case,
but if a flower-bud is a unit, are not their flowers very strange?

I have seen Dillwyn in the "Gardeners' Chronicle," and was disgusted at it,
for I thought my bilateral flowers would have been a novelty for you.

(581/4. In a letter to Hooker, dated June 2nd, 1847, Darwin makes a bold
suggestion as to floral symmetry:--)

I send you a tuft of the quasi-hybrid Laburnum, with two kinds of flowers
on same stalk, and with what strikes [me] as very curious (though I know it
has been observed before), namely, a flower bilaterally different: one
other, I observe, has half its calyx purple. Is this not very curious, and
opposed to the morphological idea that a flower is a condensed continuous
spire of leaves? Does it not look as if flowers were normally bilateral;
just in the same way as we now know that the radiating star-fish, etc., are
bilateral? The case reminds me of those insects with exactly half having
secondary male characters and the other half female.

(581/5. It is interesting to note his change of view in later years. In
an undated letter written to Mr. Spencer, probably in 1873, he says: "With
respect to asymmetry in the flowers themselves, I remain contented, from
all that I have seen, with adaptation to visits of insects. There is,
however, another factor which it is likely enough may have come into play--
viz., the protection of the anthers and pollen from the injurious effects
of rain. I think so because several flowers inhabiting rainy countries, as
A. Kerner has lately shown, bend their heads down in rainy weather.")

LETTER 582. TO J.D. HOOKER.
June [1855].

(582/1. This is an early example of Darwin's interest in the movements of
plants. Sleeping plants, as is well-known, may acquire a rhythmic movement
differing from their natural period, but the precise experiment here
described has not, as far as known, been carried out. See Pfeffer,
"Periodische Bewegungen," 1875, page 32.)

I thank you much for Hedysarum: I do hope it is not very precious, for, as
I told you, it is for probably a most foolish purpose. I read somewhere
that no plant closes its leaves so promptly in darkness, and I want to
cover it up daily for half an hour, and see if I can TEACH IT to close by
itself, or more easily than at first in darkness. I am rather puzzled
about its transmission, from not knowing how tender it is...

LETTER 583. TO J.D. HOOKER.
Down, July 19th, 1856.

I thank you warmly for the very kind manner with which you have taken my
request. It will, in truth, be a most important service to me; for it is
absolutely necessary that I should discuss single and double creations, as
a very crucial point on the general origin of species, and I must confess,
with the aid of all sorts of visionary hypotheses, a very hostile one. I
am delighted that you will take up possibility of crossing, no botanist has
done so, which I have long regretted, and I am glad to see that it was one
of A. De Candolle's desiderata. By the way, he is curiously contradictory
on subject. I am far from expecting that no cases of apparent
impossibility will be found; but certainly I expect that ultimately they
will disappear; for instance, Campanulaceae seems a strong case, but now it
is pretty clear that they must be liable to crossing. Sweet-peas (583/1.
In Lathyrus odoratus the absence of the proper insect has been supposed to
prevent crossing. See "Variation under Domestication," Edition II., Volume
II., page 68; but the explanation there given for Pisum may probably apply
to Lathyrus.), bee-orchis, and perhaps hollyhocks are, at present, my
greatest difficulties; and I find I cannot experimentise by castrating
sweet-peas, without doing fatal injury. Formerly I felt most interest on
this point as one chief means of eliminating varieties; but I feel interest
now in other ways. One general fact [that] makes me believe in my doctrine
(583/2. The doctrine which has been epitomised as "Nature abhors perpetual
self-fertilisation," and is generally known as Knight's Law or the
Knight-Darwin Law, is discussed by Francis Darwin in "Nature," 1898.
References are there given to the chief passages in the "Origin of
Species," etc., bearing on the question. See Letter 19, Volume I.), is
that NO terrestrial animal in which semen is liquid is hermaphrodite except
with mutual copulation; in terrestrial plants in which the semen is dry
there are many hermaphrodites. Indeed, I do wish I lived at Kew, or at
least so that I could see you oftener. To return again to subject of
crossing: I have been inclined to speculate so far, as to think (my!?)
notion (I say MY notion, but I think others have put forward nearly or
quite similar ideas) perhaps explains the frequent separation of the sexes
in trees, which I think I have heard remarked (and in looking over the
mono- and dioecious Linnean classes in Persoon seems true) are very apt to
have sexes separated; for [in] a tree having a vast number of flowers on
the same individual, or at least the same stock, each flower, if only
hermaphrodite on the common plan, would generally get its own pollen or
only pollen from another flower on same stock,--whereas if the sexes were
separate there would be a better chance of occasional pollen from another
distinct stock. I have thought of testing this in your New Zealand Flora,
but I have no standard of comparison, and I found myself bothered by
bushes. I should propound that some unknown causes had favoured
development of trees and bushes in New Zealand, and consequent on this
there had been a development of separation of sexes to prevent too much
intermarriage. I do not, of course, suppose the prevention of too much
intermarriage the only good of separation of sexes. But such wild notions
are not worth troubling you with the reading of.

LETTER 584. TO J.D. HOOKER.
Moor Park [May 2nd, 1857].

The most striking case, which I have stumbled on, on apparent, but false
relation of structure of plants to climate, seems to be Meyer and Doege's
remark that there is not one single, even moderately-sized, family at the
Cape of Good Hope which has not one or several species with heath-like
foliage; and when we consider this together with the number of true heaths,
any one would have been justified, had it not been for our own British
heaths (584/1. It is well known that plants with xerophytic
characteristics are not confined to dry climates; it is only necessary to
mention halophytes, alpine plants and certain epiphytes. The heaths of
Northern Europe are placed among the xerophytes by Warming ("Lehrbuch der
okologischen Pflanzengeographie," page 234, Berlin, 1896).), in saying that
heath-like foliage must stand in direct relation to a dry and moderately
warm climate. Does this not strike you as a good case of false relation?
I am so pleased with this place and the people here, that I am greatly
tempted to bring Etty here, for she has not, on the whole, derived any
benefit from Hastings. With thanks for your never failing assistance to
me...

I remember that you were surprised at number of seeds germinating in pond
mud. I tried a fourth pond, and took about as much mud (rather more than
in former case) as would fill a very large breakfast cup, and before I had
left home 118 plants had come up; how many more will be up on my return I
know not. This bears on chance of birds by their muddy feet transporting
fresh-water plants.

This would not be a bad dodge for a collector in country when plants were
not in seed, to collect and dry mud from ponds.

LETTER 585. TO ASA GRAY.
Down [1857].

I am very glad to hear that you think of discussing the relative ranges of
the identical and allied U. States and European species, when you have
time. Now this leads me to make a very audacious remark in opposition to
what I imagine Hooker has been writing (585/1. See Letter 338, Volume I.),
and to your own scientific conscience. I presume he has been urging you to
finish your great "Flora" before you do anything else. Now I would say it
is your duty to generalise as far as you safely can from your as yet
completed work. Undoubtedly careful discrimination of species is the
foundation of all good work; but I must look at such papers as yours in
Silliman as the fruit. As careful observation is far harder work than
generalisation, and still harder than speculation, do you not think it very
possible that it may be overvalued? It ought never to be forgotten that
the observer can generalise his own observations incomparably better than
any one else. How many astronomers have laboured their whole lives on
observations, and have not drawn a single conclusion; I think it is
Herschel who has remarked how much better it would be if they had paused in
their devoted work and seen what they could have deduced from their work.
So do pray look at this side of the question, and let us have another paper
or two like the last admirable ones. There, am I not an audacious dog!

You ask about my doctrine which led me to expect that trees would tend to
have separate sexes. I am inclined to believe that no organic being exists
which perpetually self-fertilises itself. This will appear very wild, but
I can venture to say that if you were to read my observations on this
subject you would agree it is not so wild as it will at first appear to
you, from flowers said to be always fertilised in bud, etc. It is a long
subject, which I have attended to for eighteen years. Now, it occurred to
me that in a large tree with hermaphrodite flowers, we will say it would be
ten to one that it would be fertilised by the pollen of its own flower, and
a thousand or ten thousand to one that if crossed it would be crossed only
with pollen from another flower of same tree, which would be opposed to my
doctrine. Therefore, on the great principle of "Nature not lying," I fully
expected that trees would be apt to be dioecious or monoecious (which, as
pollen has to be carried from flower to flower every time, would favour a
cross from another individual of the same species), and so it seems to be
in Britain and New Zealand. Nor can the fact be explained by certain
families having this structure and chancing to be trees, for the rule seems
to hold both in genera and families, as well as in species.

I give you full permission to laugh your fill at this wild speculation; and
I do not pretend but what it may be chance which, in this case, has led me
apparently right. But I repeat that I feel sure that my doctrine has more
probability than at first it appears to have. If you had not asked, I
should not have written at such length, though I cannot give any of my
reasons.

The Leguminosae are my greatest opposers: yet if I were to trust to
observations on insects made during many years, I should fully expect
crosses to take place in them; but I cannot find that our garden varieties
ever cross each other. I do NOT ask you to take any trouble about it, but
if you should by chance come across any intelligent nurseryman, I wish you
would enquire whether they take any pains in raising the varieties of
papilionaceous plants apart to prevent crossing. (I have seen a statement
of naturally formed crossed Phaseoli near N. York.) The worst is that
nurserymen are apt to attribute all varieties to crossing.

Finally I incline to believe that every living being requires an occasional
cross with a distinct individual; and as trees from the mere multitude of
flowers offer an obstacle to this, I suspect this obstacle is counteracted
by tendency to have sexes separated. But I have forgotten to say that my
maximum difficulty is trees having papilionaceous flowers: some of them, I
know, have their keel-petals expanded when ready for fertilisation; but
Bentham does not believe that this is general: nevertheless, on principle
of nature not lying, I suspect that this will turn out so, or that they are
eminently sought by bees dusted with pollen. Again I do NOT ask you to
take trouble, but if strolling under your Robinias when in full flower,
just look at stamens and pistils whether protruded and whether bees visit
them. I must just mention a fact mentioned to me the other day by Sir W.
Macarthur, a clever Australian gardener: viz., how odd it was that his
Erythrinas in N.S. Wales would not set a seed, without he imitated the
movements of the petals which bees cause. Well, as long as you live, you
will never, after this fearfully long note, ask me why I believe this or
that.

LETTER 586. TO ASA GRAY.
June 18th [1857].

It has been extremely kind of you telling me about the trees: now with
your facts, and those from Britain, N. Zealand, and Tasmania I shall have
fair materials for judging. I am writing this away from home, but I think
your fraction of 95/132 is as large as in other cases, and is at least a
striking coincidence.

I thank you much for your remarks about my crossing notions, to which, I
may add, I was led by exactly the same idea as yours, viz., that crossing
must be one means of eliminating variation, and then I wished to make out
how far in animals and vegetables this was possible. Papilionaceous
flowers are almost dead floorers to me, and I cannot experimentise, as
castration alone often produces sterility. I am surprised at what you say
about Compositae and Gramineae. From what I have seen of latter they
seemed to me (and I have watched wheat, owing to what L. de Longchamps has
said on their fertilisation in bud) favourable for crossing; and from
Cassini's observations and Kolreuter's on the adhesive pollen, and C.C.
Sprengel's, I had concluded that the Compositae were eminently likely (I am
aware of the pistil brushing out pollen) to be crossed. (586/1. This is
an instance of the curious ignorance of the essential principles of floral
mechanism which was to be found even among learned and accomplished
botanists such as Gray, before the publication of the "Fertilisation of
Orchids." Even in 1863 we find Darwin explaining the meaning of dichogamy
in a letter to Gray.) If in some months' time you can find time to tell me
whether you have made any observations on the early fertilisation of plants
in these two orders, I should be very glad to hear, as it would save me
from great blunder. In several published remarks on this subject in
various genera it has seemed to me that the early fertilisation has been
inferred from the early shedding of the pollen, which I think is clearly a
false inference. Another cause, I should think, of the belief of
fertilisation in the bud, is the not-rare, abnormal, early maturity of the
pistil as described by Gartner. I have hitherto failed in meeting with
detailed accounts of regular and normal impregnation in the bud.
Podostemon and Subularia under water (and Leguminosae) seem and are
strongest cases against me, as far as I as yet know. I am so sorry that
you are so overwhelmed with work; it makes your VERY GREAT kindness to me
the more striking.

It is really pretty to see how effectual insects are. A short time ago I
found a female holly sixty measured yards from any other holly, and I cut
off some twigs and took by chance twenty stigmas, cut off their tops, and
put them under the microscope: there was pollen on every one, and in
profusion on most! weather cloudy and stormy and unfavourable, wind in
wrong direction to have brought any.

LETTER 587. TO J.D. HOOKER.
Down, January 12th [1858].

I want to ask a question which will take you only few words to answer. It
bears on my former belief (and Asa Gray strongly expressed opinion) that
Papilionaceous flowers were fatal to my notion of there being no eternal
hermaphrodites. First let me say how evidence goes. You will remember my
facts going to show that kidney-beans require visits of bees to be
fertilised. This has been positively stated to be the case with Lathyrus
grandiflorus, and has been very partially verified by me. Sir W. Macarthur
tells me that Erythrina will hardly seed in Australia without the petals
are moved as if by bee. I have just met the statement that, with common
bean, when the humble-bees bite holes at the base of the flower, and
therefore cease visiting the mouth of the corolla, "hardly a bean will
set." But now comes a much more curious statement, that [in] 1842-43,
"since bees were established at Wellington (New Zealand), clover seeds all
over the settlement, WHICH IT DID NOT BEFORE." (587/1. See Letter 362,
Volume I.) The writer evidently has no idea what the connection can be.
Now I cannot help at once connecting this statement (and all the foregoing
statements in some degree support each other, as all have been advanced
without any sort of theory) with the remarkable absence of Papilionaceous
plants in N. Zealand. I see in your list Clianthus, Carmichaelia (four
species), a new genus, a shrub, and Edwardsia (is latter Papilionaceous?).
Now what I want to know is whether any of these have flowers as small as
clover; for if they have large flowers they may be visited by humble-bees,
which I think I remember do exist in New Zealand; and which humble-bees
would not visit the smaller clover. Even the very minute little yellow
clover in England has every flower visited and revisited by hive-bees, as I
know by experience. Would it not be a curious case of correlation if it
could be shown to be probable that herbaceous and small Leguminosae do not
exist because when [their] seeds [are] washed ashore (!!!) no small bees
exist there. Though this latter fact must be ascertained. I may not prove
anything, but does it not seem odd that so many quite independent facts, or
rather statements, should point all in one direction, viz., that bees are
necessary to the fertilisation of Papilionaceous flowers?

LETTER 588. TO JOHN LUBBOCK (Lord Avebury).
Sunday [1859].

Do you remember calling my attention to certain flowers in the truss of
Pelargoniums not being true, or not having the dark shade on the two upper
petals? I believe it was Lady Lubbock's observation. I find, as I
expected, it is always the central or sub-central flower; but what is far
more curious, the nectary, which is blended with the peduncle of the
flowers, gradually lessens and quite disappears (588/1. This fact is
mentioned in Maxwell Masters' "Vegetable Teratology" (Ray Society's
Publications), 1869, page 221.), as the dark shade on the two upper petals
disappears. Compare the stalk in the two enclosed parcels, in each of
which there is a perfect flower.

Now, if your gardener will not be outrageous, do look over your geraniums
and send me a few trusses, if you can find any, having the flowers without
the marks, sending me some perfect flowers on same truss. The case seems
to me rather a pretty one of correlation of growth; for the calyx also
becomes slightly modified in the flowers without marks.

LETTER 589. TO MAXWELL MASTERS.
Down, April 7th [1860].

I hope that you will excuse the liberty which I take in writing to you and
begging a favour. I have been very much interested by the abstract (too
brief) of your lecture at the Royal Institution. Many of the facts alluded
to are full of interest for me. But on one point I should be infinitely
obliged if you could procure me any information: namely, with respect to
sweet-peas. I am a great believer in the natural crossing of individuals
of the same species. But I have been assured by Mr. Cattell (589/1. The
nurseryman he generally dealt with.), of Westerham, that the several
varieties of sweet-pea can be raised close together for a number of years
without intercrossing. But on the other hand he stated that they go over
the beds, and pull up any false plant, which they very naturally attribute
to wrong seeds getting mixed in the lot. After many failures, I succeeded
in artificially crossing two varieties, and the offspring out of the same
pod, instead of being intermediate, was very nearly like the two pure
parents; yet in one, there was a trace of the cross, and these crossed peas
in the next generation showed still more plainly their mongrel origin.
Now, what I want to know is, whether there is much variation in sweet-peas
which might be owing to natural crosses. What I should expect would be
that they would keep true for many years, but that occasionally, perhaps at
long intervals, there would be a considerable amount of crossing of the
varieties grown close together. Can you give, or obtain from your father,
any information on this head, and allow me to quote your authority? It
would really be a very great favour and kindness.

LETTER 590. TO J.D. HOOKER.

(590/1. The genera Scaevola and Leschenaultia, to which the following
letter refers, belong to the Goodeniaceae (Goodenovieae, Bentham & Hooker),
an order allied to the Lobeliaceae, although the mechanism of fertilisation
resembles rather more nearly that of Campanula. The characteristic feature
of the flower in this order is the indusium, or, as Delpino (590/2.
Delpino's observations on Dichogamy, summarised by Hildebrand in "Bot.
Zeitung," 1870, page 634.) calls it, the "collecting cup": this cuplike
organ is a development of the style, and serves the same function as the
hairs on the style of Campanula, namely, that of taking the pollen from the
anthers and presenting it to the visiting insect. During this stage the
immature stigma is at the bottom of the cup, and though surrounded by
pollen is incapable of being pollinated. In most genera of the order the
pollen is pushed out of the indusium by the growth of the style or stigma,
very much as occurs in Lobelia or the Compositae. Finally the style
emerges from the indusium (590/3. According to Hamilton ("Proc. Linn. Soc.
N. S. Wales," X., 1895, page 361) the stigma rarely grows beyond the
indusium in Dampiera. In the same journal (1885-6, page 157, and IX.,
1894, page 201) Hamilton has given a number of interesting observations on
Goodenia, Scaevola, Selliera, Brunonia. There seem to be mechanisms for
cross- and also for self-fertilisation.), the stigmas open out and are
pollinated from younger flowers. The mechanism of fertilisation has been
described by F. Muller (590/4. In a letter to Hildebrand published in the
"Bot. Zeitung," 1868, page 113.), and more completely by Delpino (loc.
cit.).