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.

[Sidenote: 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.”

This tendency must apply to species as well as individuals, the best adapted becoming abundant, the others scarce or even extinct. If we knew the whole of the conditions and powers of a species “we might be able even to calculate the proportionate abundance of individuals, which is the necessary result.”

Hence, first, _the animal population of a country is generally stationary (due to food and other checks)_; second, _comparative abundance or scarcity of individuals is entirely due to organisation and resulting habits, the varying measure of success in the struggle being balanced by a varying population in a given area_.

Variations from type must nearly always affect habits or capacities. Even changes of colour may promote concealment, while changes in the limbs or any external organs would affect the mode of procuring food, etc. “An antelope with shorter or weaker legs must necessarily suffer more from the attacks of the feline carnivora”; the passenger pigeon with less powerful wings could not always procure sufficient food. Hence species thus modified would gradually diminish; but, on the other hand, if modified in the direction of increased powers, would become more numerous. Varieties will fall under these two classes--those which will never rival, and those which will eventually outnumber, the parent species. If, then, some alteration in conditions occurred making existence more difficult to a certain species, first the less favourable variety would suffer and become extinct, then the parent species, while the superior variety would alone remain, “and on a return to favourable circumstances would rapidly increase in numbers and occupy the place of the extinct species and variety.”

The superior _variety_ would thus replace the _species_, to which it _could not_ return, for the latter could never compete with the former. Hence a tendency to revert would be checked. But the superior variety, when established, would in time give rise to new varieties, some of which would become predominant. Hence _progression and continued divergence_ would follow, but not invariably, for the criteria of success or failure would vary, and would sometimes render a race which was under other conditions the most favoured now the least so. Variations without any effect on the life-preserving powers might also occur. But it is contended that certain varieties must, on the average, tend to persist longer than the parent species, while the scale on which nature works is so vast that an average tendency must in the end attain its full result.

Comparing domestic with wild animals, the very existence of the latter depends upon their senses and physical powers. Not so with the former, which are defended and fed by man.

Any favourable variety of a domestic animal is utterly useless to itself; while any increase of the powers and faculties of wild animals is immediately available, creating, as it were, a new and superior animal.

Again, with domestic animals all variations have an equal chance, and those which would be extremely injurious in a wild state are, under the artificial conditions, no disadvantage. Our domestic breeds could never have come into existence in a wild state, and if turned wild “_must_ return to something near the type of the original wild stock, _or become altogether extinct_.”[E]

Hence we cannot argue from domestic to wild animals, the conditions of life in the two being completely opposed.

Lamarck’s hypothesis of change produced by the attempts of animals to increase the development of their own organs has been often refuted, but the view here proposed depends upon the action of principles constantly working in nature. Retractile talons of falcons and cats have not been developed by volition, but by _the survival of those which had the greatest facilities for seizing prey_. The long neck of the giraffe was not produced by constant stretching, but by the success which any increase in the length of neck ensured to its possessors. Even colours, especially of insects, are explained in the same way, for among the varieties of many tints, those “having colours best adapted to concealment ... would inevitably survive the longest.” We can similarly explain deficiency of some organs with compensating development of others, “great velocity making up for the absence of defensive weapons,” etc. Varieties with an unbalanced deficiency could not long survive. The action of the principle is like the governor of a steam-engine, checking irregularities almost before they become evident. Such a view accords well with “the many lines of divergence from a central type”; the increasing efficiency of a particular organ in a series of allied species; the persistence of unimportant parts when important ones have changed; the “more specialised structure,” said by Owen to be characteristic of recent as compared with extinct forms.

Hence there is a tendency of certain classes of _varieties_ to progress further and further from the original type, and there is no reason for assigning any limit to this progression. Such gradual changes “may, it is believed, be followed out so as to agree with all the phenomena presented by organised beings, their extinction and succession in past ages, and all the extraordinary modifications of form, instinct, and habits which they exhibit.”

Wallace’s Essay has been reprinted without any alteration in his “Essays on Natural Selection,” recently re-issued combined with “Tropical Nature.”