CHAPTER 2.XI.
THE 'EFFECTS OF CROSS- AND SELF-FERTILISATION IN THE VEGETABLE KINGDOM.'
1876.
[This book, as pointed out in the 'Autobiography,' is a complement to the
'Fertilisation of Orchids,' because it shows how important are the results
of cross-fertilisation which are ensured by the mechanisms described in
that book.
By proving that the offspring of cross-fertilisation are more vigorous than
the offspring of self-fertilisation, he showed that one circumstance which
influences the fate of young plants in the struggle for life is the degree
to which their parents are fitted for cross-fertilisation. He thus
convinced himself that the intensity of the struggle (which he had
elsewhere shown to exist among young plants) is a measure of the strength
of a selective agency perpetually sifting out every modification in the
structure of flowers which can effect its capabilities for cross-
fertilisation.
The book is also valuable in another respect, because it throws light on
the difficult problems of the origin of sexuality. The increased vigour
resulting from cross-fertilisation is allied in the closest manner to the
advantage gained by change of conditions. So strongly is this the case,
that in some instances cross-fertilisation gives no advantage to the
offspring, unless the parents have lived under slightly different
conditions. So that the really important thing is not that two individuals
of different BLOOD shall unite, but two individuals which have been
subjected to different conditions. We are thus led to believe that
sexuality is a means for infusing vigour into the offspring by the
coalescence of differentiated elements, an advantage which could not follow
if reproductions were entirely asexual.
It is remarkable that this book, the result of eleven years of experimental
work, owed its origin to a chance observation. My father had raised two
beds of Linaria vulgaris--one set being the offspring of cross- and the
other of self-fertilisation. These plants were grown for the sake of some
observations on inheritance, and not with any view to cross-breeding, and
he was astonished to observe that the offspring of self-fertilisation were
clearly less vigorous than the others. It seemed incredible to him that
this result could be due to a single act of self-fertilisation, and it was
only in the following year when precisely the same result occurred in the
case of a similar experiment on inheritance in Carnations, that his
attention was "thoroughly aroused" and that he determined to make a series
of experiments specially directed to the question. The following letters
give some account of the work in question.]
CHARLES DARWIN TO ASA GRAY.
September 10, [1866?].
...I have just begun a large course of experiments on the germination of
the seed, and on the growth of the young plants when raised from a pistil
fertilised by pollen from the same flower, and from pollen from a distinct
plant of the same, or of some other variety. I have not made sufficient
experiments to judge certainly, but in some cases the difference in the
growth of the young plants is highly remarkable. I have taken every kind
of precaution in getting seed from the same plant, in germinating the seed
on my own chimney-piece, in planting the seedlings in the same flower-pot,
and under this similar treatment I have seen the young seedlings from the
crossed seed exactly twice as tall as the seedlings from the self-
fertilised seed; both seeds having germinated on the same day. If I can
establish this fact (but perhaps it will all go to the dogs), in some fifty
cases, with plants of different orders, I think it will be very important,
for then we shall positively know why the structure of every flower
permits, or favours, or necessitates an occasional cross with a distinct
individual. But all this is rather cooking my hare before I have caught
it. But somehow it is a great pleasure to me to tell you what I am about.
Believe me, my dear Gray,
Ever yours most truly, and with cordial thanks,
CH. DARWIN.
CHARLES DARWIN TO G. BENTHAM.
April 22, 1868.
...I am experimenting on a very large scale on the difference in power of
growth between plants raised from self-fertilised and crossed seeds; and it
is no exaggeration to say that the difference in growth and vigour is
sometimes truly wonderful. Lyell, Huxley and Hooker have seen some of my
plants, and been astonished; and I should much like to show them to you. I
always supposed until lately that no evil effects would be visible until
after several generations of self-fertilisation; but now I see that one
generation sometimes suffices; and the existence of dimorphic plants and
all the wonderful contrivances of orchids are quite intelligible to me.
With cordial thanks for your letter, which has pleased me greatly,
Yours very sincerely,
CHARLES DARWIN.
[An extract from a letter to Dr. Gray (March 11, 1873) mentions the
progress of the work:--
"I worked last summer hard at Drosera, but could not finish till I got
fresh plants, and consequently took up the effects of crossing and self-
fertilising plants, and am got so interested that Drosera must go to the
dogs till I finish with this, and get it published; but then I will resume
my beloved Drosera, and I heartily apologise for having sent the precious
little things even for a moment to the dogs."
The following letters give the author's impression of his own book.]
CHARLES DARWIN TO J. MURRAY.
Down, September 16, 1876.
My dear Sir,
I have just received proofs in sheet of five sheets, so you will have to
decide soon how many copies will have to be struck off. I do not know what
to advise. The greater part of the book is extremely dry, and the whole on
a special subject. Nevertheless, I am convinced that the book is of value,
and I am convinced that for MANY years copies will be occasionally sold.
Judging from the sale of my former books, and from supposing that some
persons will purchase it to complete the set of my works, I would suggest
1500. But you must be guided by your larger experience. I will only
repeat that I am convinced the book is of some permanent value...
CHARLES DARWIN TO VICTOR CARUS.
Down, September 27, 1876.
My dear Sir,
I sent by this morning's post the four first perfect sheets of my new book,
the title of which you will see on the first page, and which will be
published early in November.
I am sorry to say that it is only shorter by a few pages than my
'Insectivorous Plants.' The whole is now in type, though I have corrected
finally only half the volume. You will, therefore, rapidly receive the
remainder. The book is very dull. Chapters II. to VI., inclusive, are
simply a record of experiments. Nevertheless, I believe (though a man can
never judge his own books) that the book is valuable. You will have to
decide whether it is worth translating. I hope so. It has cost me very
great labour, and the results seem to me remarkable and well established.
If you translate it, you could easily get aid for Chapters II. to VI., as
there is here endless, but I have thought necessary repetition. I shall be
anxious to hear what you decide...
I most sincerely hope that your health has been fairly good this summer.
My dear Sir, yours very truly,
CH. DARWIN.
CHARLES DARWIN TO ASA GRAY.
Down, October 28, 1876.
My dear Gray,
I send by this post all the clean sheets as yet printed, and I hope to send
the remainder within a fortnight. Please observe that the first six
chapters are not readable, and the six last very dull. Still I believe
that the results are valuable. If you review the book, I shall be very
curious to see what you think of it, for I care more for your judgment than
for that of almost any one else. I know also that you will speak the
truth, whether you approve or disapprove. Very few will take the trouble
to read the book, and I do not expect you to read the whole, but I hope you
will read the latter chapters.
...I am so sick of correcting the press and licking my horrid bad style
into intelligible English.
[The 'Effects of Cross and Self-fertilisation' was published on November
10, 1876, and 1500 copies were sold before the end of the year. The
following letter refers to a review in 'Nature' (February 15, 1877.):]
CHARLES DARWIN TO W. THISELTON DYER.
Down, February 16, 1877.
Dear Dyer,
I must tell you how greatly I am pleased and honoured by your article in
'Nature,' which I have just read. You are an adept in saying what will
please an author, not that I suppose you wrote with this express intention.
I should be very well contented to deserve a fraction of your praise. I
have also been much interested, and this is better than mere pleasure, by
your argument about the separation of the sexes. I dare say that I am
wrong, and will hereafter consider what you say more carefully: but at
present I cannot drive out of my head that the sexes must have originated
from two individuals, slightly different, which conjugated. But I am aware
that some cases of conjugation are opposed to any such views.
With hearty thanks,
Yours sincerely,
CHARLES DARWIN.
