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.
CHAPTER X.
WALLACE’S SECTION OF THE JOINT MEMOIR READ BEFORE THE LINNEAN SOCIETY JULY 1, 1858.
The communication by Alfred Russel Wallace was entitled “On the Tendency of Varieties to depart indefinitely from the Original Type.” An abstract of it is given below.
WALLACE’S ESSAY.
Varieties produced in domesticity are more or less unstable, and often tend to return to the parent form. This is usually thought to be true for all varieties, and to be a strong argument for the original and permanent distinctness of species.
On the other hand, races forming “permanent or true varieties” are well known, and there are generally no means of determining which is the variety and which the original species. The hypothesis of a “permanent invariability of species” is satisfied by supposing that, while such varieties cannot diverge from the species beyond a certain fixed limit, they may return to it.
This argument is founded on the assumption that varieties in nature are in all respects identical with those of domestic animals. The object of the paper is to show that this is false, and “that there is a general principle in nature which will cause many varieties to survive the parent species and to give rise to successive variations departing further and further from the original type.” The same principle explains the tendency of domestic animals to return to the parent form.
“The life of wild animals is a struggle for existence.” To procure food and escape enemies are the primary conditions of existence, and determine abundance and rarity, frequently seen in closely allied species.
“Large animals cannot be so abundant as small ones; the carnivora must be less numerous than the herbivora,” eagles and lions than pigeons and antelopes. Fecundity has little or nothing to do with this. The least prolific animals would increase rapidly if unchecked. But wild animals do not increase beyond their average; hence there must be an immense amount of destruction. The abundance of species in individuals bears no relation whatever to their fertility. Thus the excessively abundant passenger pigeon of the United States lays only one or two eggs. Its abundance is explained by the widespread supply of food rendered available by its powers of flight. The food-supply “is almost the sole condition requisite for ensuring the rapid increase of a given species.” This explains why the sparrow is more abundant than the red-breast, why aquatic species of birds are specially numerous in individuals, why the wild cat is rarer than the rabbit. “So long as a country remains physically unchanged, the numbers of its animal population cannot materially increase.” If one species does so, others must diminish. In the immense amount of destruction the weakest must die, “while those that prolong their existence can only be the most perfect in health and vigour—those who are best able to obtain food regularly and to avoid their numerous enemies. It is, as we commenced by remarking, ‘a struggle for existence,’ in which the weakest and least perfectly organised must always succumb.”