Charles Darwin and the Theory of Natural Selection

by Sir Edward Bagnall Poulton

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.”