The title of the joint paper was “On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection.” It was read July 1st, 1858.
CHAPTER IX.
DARWIN’S SECTION OF THE JOINT MEMOIR READ BEFORE THE LINNEAN SOCIETY JULY 1, 1858.
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.