Evolution and Adaptation

by Thomas Hunt Morgan

First Law. In every animal, that has not passed beyond the term of its development, the frequent and sustained use of any organ strengthens it, develops it, increases its size, and gives it strength proportionate to the length of time of its employment. On the other hand, the continued lack of use of the same organ sensibly weakens it; it deteriorates, and its faculties diminish progressively until at last it disappears.

Second Law. Nature preserves everything that she has caused the individual to acquire or to lose by the influence of the circumstances to which the race has been for a long time exposed, and consequently by the influence of the predominant use of certain organs (or in consequence of its continued disuse). She does this by the generation of new individuals which are produced with the newly acquired organs. This occurs, provided that the acquired changes were common to the two sexes, or to the individuals that produced the new forms.

These laws are, Lamarck says, fundamental truths which cannot be misunderstood except by those who have never observed or followed nature in her operations. He insists that it is a mistake to suppose that the parts are responsible for the functions, for it is easy to demonstrate that it is the needs and uses of the organs that have caused the parts to develop.

If it is supposed, he continues, that these laws are hypothetical, they may be demonstrated by the following facts: The adult baleen whale is without teeth, although in the fœtus teeth are present, concealed in the jaws. The loss of the teeth is the result of the whale swallowing its food without first masticating it. The ant-eater is also without teeth, and has also the habit of swallowing its food without chewing it. The mole has very small eyes, and this is the result of its having made very little use of them, since its habits are subterranean. Another animal, the aspalax, has only the rudiments of eyes, and has almost completely lost the power of sight. This animal also lives underground like the mole.

Proteus, an aquatic salamander living in deep caves, has only rudimentary eyes. In these latter cases it is the disuse of the eye that has led to its degeneration. This is proven, Lamarck adds, by the fact that the organs of hearing are never in this condition, because sound vibrations penetrate everywhere, even into the densest bodies.

It is a part of the plan of organization of the reptiles that they have four legs; but the snakes, although belonging to this group, have no legs. This absence of legs is explained by their having acquired the habit of gliding over the ground, and of concealing themselves in the grass. Owing to their repeated effort to elongate themselves, in order to pass through narrow spaces, their bodies have become drawn out. Under these circumstances legs would be useless, since long ones would interfere with their motion, and short ones could not move their long bodies. Since the plan of organization limits the snakes to only four legs, and since this number would be useless, they have disappeared.

Many insects are destitute of wings, although wings are a part of the plan of organization of this group. They are absent only in those forms whose habits render wings useless, consequently they have disappeared through disuse.

The preceding cases are those in which the disuse of an organ has led to its degeneration. The following cases are cited to show that by use an organ increases in size. The formation of the web in the feet of water-birds has already been given as a character which Lamarck supposes to have been acquired through use; also the case of shore-birds, which, by an effort to elongate their legs, have actually made them so in the course of time. The necks of water-birds are also long on account of their having been stretched in the efforts to catch fish. The long tongues of the ant-eater, of the woodpecker, and of humming-birds are the result of use, and the long, forked tongue of serpents has come from their using their tongue to feel objects in front of them.

Fishes that have acquired the habit of living in shallow water, flounders, soles, etc., have been forced to swim on their sides in order to approach nearer to the shore. Since more light comes from above than from below, the eye on the under side, straining to turn to the light, has finally migrated to the upper side.

The habit of eating great quantities of food, which distends the digestive organs, has caused the bodies of herbivorous quadrupeds to become large, as seen in the elephant, the rhinoceros, oxen, horses, and buffaloes. The habit of standing for a long time on their feet has caused some animals to develop hard, thick hoofs. Herbivorous animals, that inhabit countries where they are constantly subjected to attack, as deer and antelopes for example, are forced to escape by rapid flight, and in consequence their bodies have become slenderer and their legs thinner. The horns, antlers, and protuberances that many of these animals possess are the results of their butting each other when angered.