CHAPTER IX.

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.