DARWIN’S SECTION OF THE JOINT MEMOIR READ BEFORE THE LINNEAN SOCIETY
JULY 1, 1858.
[Sidenote: FIRST PUBLISHED ESSAY.]
The first section of Darwin’s communication consisted of extracts
from the Second Chapter of the First Part of his manuscript essay of
1844. The Part was entitled “The Variation of Organic Beings under
Domestication, and in their Natural State,” and the Second Chapter was
headed “On the Variation of Organic Beings in a State of Nature; on
the Natural Means of Selection; on the Comparison of Domestic Races
and True Species.” The extracts first deal with the tendency towards
rapid multiplication and the consequent struggle for life. The average
constancy of the numbers of individuals is traced to the average
constancy of the amount of food, “whereas the increase of all organisms
tends to be geometrical.” Practical illustrations are given in the
enormous increase of the mice in La Plata during the drought which
killed millions of cattle, and in the well-known and rapid increase
of the animals and plants introduced by man into a new and favourable
country.
The checks which operate when the country is stocked and the species
reaches its average are most difficult to detect, but none the less
certain. If any check is lightened in the case of any organism it will
at once tend to increase. “Nature may be compared to a surface on which
rest ten thousand sharp wedges touching each other and driven inward by
incessant blows.” Darwin meant by this image to express that just as
any single wedge would instantly rise above the rest when the blows on
it were in any way lessened as compared with those on the other wedges,
so it would be with the proportionate number of any species when the
checks to which it is subjected are in any way relaxed.
If the external conditions alter, and the changes continue progressing,
the inhabitants will be less well adapted than formerly. The changed
conditions would act on the reproductive system and render the
organisation plastic. Now, can it be doubted, from the struggle each
individual has to obtain subsistence, that any minute variation in
structure, habits, or instincts adapting that individual better to the
new conditions would tell upon its vigour and health? “Yearly more are
bred than can survive; the smallest grain in the balance, in the long
run, must tell on which death shall fall, and which shall survive.” If
this went on for a thousand generations who will deny its effect “when
we remember what, in a few years, Bakewell effected in cattle, and
Western in sheep, by this identical principle of selection?”
He gives an imaginary example of a canine animal preying on rabbits and
hares. If the rabbits, constituting its chief food, gradually became
rarer, and the hares more plentiful, the animal would be driven to
try and catch more hares, and hence would be selected in the direction
of speed and sharp eyesight. “I can see no more reason to doubt that
these cases in a thousand generations would produce a marked effect,
and adapt the form of the fox or dog to the catching of hares instead
of rabbits, than that greyhounds can be improved by selection and
careful breeding.” So also with plants having seeds with rather more
down, leading to wider dissemination. Darwin here added this note: “I
can see no more difficulty in this, than in the planter improving his
varieties of the cotton plant. C. D. 1858.”
Then follows a brief sketch of sexual selection and a comparison
with natural selection, and the conclusion is reached--“this kind
of selection, however, is less vigorous than the other; it does not
require the death of the less successful, but gives to them fewer
descendants. The struggle falls, moreover, at a time of year when food
is generally abundant, and perhaps the effect chiefly produced would
be the modification of the secondary sexual characters, which are not
related to the power of obtaining food, or to defence from enemies, but
to fighting with or rivalling other males.”
The second section was entitled “Abstract of a Letter from C. Darwin,
Esq., to Professor Asa Gray, Boston, U.S., dated Down, September 5th,
1857.” To this letter Darwin attached great importance as a convenient
and brief account of the essentials of his theory, written and sent to
Asa Gray many months before he received Wallace’s essay. A tolerably
full abstract of the letter, which is itself a very brief abstract,
is therefore printed below. The epitome here given is taken from the
letter itself, and is in certain respects more full than that published
in the Linnean Journal.
In the introductory parts Darwin explained that “the facts which
kept me longest scientifically orthodox are those of adaptation--the
pollen-masses in asclepias--the mistletoe, with its pollen carried by
insects, and seed by birds--the woodpecker, with its feet and tail,
beak and tongue, to climb the tree and secure insects. To talk of
climate or Lamarckian habit producing such adaptations to other organic
beings is futile. This difficulty I believe I have surmounted.” Having
then stated that the reasons which induced him to accept evolution
were “general facts in the affinities, embryology, rudimentary organs,
geological history, and geographical distribution of organic beings,”
he proceeds to give a brief account of his “notions on the means by
which Nature makes her species.” The following is an abstract of the
account he gives:--
[Sidenote: SUMMARY OF THE ESSAY.]
I. The success with which selection has been applied by man in
making his breeds of domestic animals and plants: and this even
in ancient times when the selection was unconscious, viz. when
breeding was not thought of, but the most useful animals and
plants were kept and the others destroyed. “Selection acts only
by the accumulation of very slight or greater variations,” and
man in thus accumulating “_may be said_ to make the wool of one
sheep good for carpets, and another for cloth, &c.”
II. Slight variations of all parts of the organism occur in
nature, and if a being could select with reference to the whole
structure, what changes might he not effect in the almost
unlimited time of which geology assures us.
III. Animals increase so fast that, but for extermination,
the earth would not hold the progeny of even the slowest
breeding animal. Only a few in each generation can live; hence
the struggle for life, which has never yet been sufficiently
appreciated. “What a trifling difference must often determine
which shall survive and which perish!” Thus is supplied the
“unerring power” of “_Natural Selection_ ... which selects
exclusively for the good of each organic being.”
IV. If a country were changing the altered conditions would tend
to cause variation, “not but what I believe most beings vary at
all times enough for selection to act on.” Extermination would
expose the remainder to “the mutual action of a different set of
inhabitants, which I believe to be more important to the life
of each being than mere climate.” In the infinite complexity
of the struggle for life “I cannot doubt that during millions
of generations individuals of a species will be born with
some slight variation profitable to some part of its economy;
such will have a better chance of surviving and propagating
this variation, which again will be slowly increased by the
accumulative action of natural selection; and the variety
thus formed will either coexist with, or more commonly will
exterminate its parent form.” Thus complex adaptations like those
of woodpecker or mistletoe may be produced.
V. Numerous difficulties can be answered satisfactorily in time.
The supposed changes are only very gradual, and very slow, “only
a few undergoing change at any one time.” The imperfection of
the geological record accounts for deficient direct evidence of
change.
VI. Divergence during evolution will be an advantage. “The
same spot will support more life if occupied by very diverse
forms.” Hence during the increase of species into its
offspring--varieties, or sub-species, or true species, the latter
“will try (only few will succeed) to seize on as many and as
diverse places in the economy of nature as possible,” and so
will tend to “exterminate its less well-fitted parent.” This
explains classification, in which the organic beings “always
_seem_ to branch and sub-branch like a tree from a common trunk;
the flourishing twigs destroying the less vigorous--the dead and
lost branches rudely representing extinct genera and families.”
In a postscript he says:--
“This little abstract touches only the accumulative power of
natural selection, which I look at as by far the most important
element in the production of new forms. The laws governing the
incipient or primordial variation (unimportant except as the
groundwork for selection to act on, in which respect it is all
important), I shall discuss under several heads, but I can come,
as you may well believe, only to very partial and imperfect
conclusions.”
It is, I think, of especial interest to find Darwin at this early
period arguing in a most convincing manner for the creative power
of natural selection. The selective power becomes, by accumulation,
of such paramount importance in the process, as compared with the
variations, that, although these latter are absolutely essential, man
may be said to _make_ his domestic breeds and Nature her species. The
man who argued thus had been through and had left behind the difficulty
that, even now, is often raised--that “before anything can be selected
it must be,” and therefore that selection is of small account as
compared with variation.