Favourable times and circumstances

In my ‘Research Into Living Bodies (p. 50)’ I laid down the following proposition:

It is not the organs, that is, the nature and the form of the animal’s body parts, which cause habits and special faculties. In contrast, its habits, manner of life, and circumstances of the individuals from which the animal comes to possess, over time, the form of its body, the number and condition of its organs, and finally the faculties which it enjoys.

Lamarck

Favourable times and circumstances are the 2 main means employed by nature to bring into existence all her productions.

Time has no limits for her. She always has time to spare.

Circumstances for her are inexhaustible.

The main circumstances arise from the influence of:

• climates and various temperatures in the environment
• the variety of places and their exposure
• habits
• the most ordinary movements
• the most frequent actions
• the means of self-preservation, reproduction, etc

These various influences cause the faculties to expand and grow stronger through use.

With new habits preserved over a long time they diversify.

Imperceptibly the nature and the condition of the parts and the organs undergo the consequences of all these influences.

These preserve and propagate themselves in reproduction.

These truths are the consequences of the 2 natural laws above.

The constant lack of exercise of an organ:

• reduces its faculties
• then it gradually shrinks it
• then it makes it disappear or even destroys it

Then I will reveal how, by contrast, the habit of exercising an organ, in every animal which has not reached the limit in the diminution of its faculties, not only improves this organ’s faculties and makes it grow, but also makes it develop and acquire dimensions which imperceptibly change it, so that in time it makes it quite different from the same organ examined in another animal which exercises it much less.

The lack of use of an organ, once it has become constant because of the habits which one has taken up, gradually diminishes that organ and ends up by making it disappear and even destroying it.

Similar vertebrate animals have much diversity in their parts.

• Yet all have their jaws equipped with teeth.

In animals that swallow instead of chew, their teeth have not undergone any development.

They have either:

• remained hidden between the bony layers of the jaws without appearing on the outside or
• have even been destroyed

We thought that the whale entirely has no teeth.

Geoffroy has found them hidden in the jaws of the fetus of the animal.

He has also located in birds the groove where the teeth were before.

• But we do not see them there any more.

Many vertebrates have eyes in the head.

Nevertheless, the mole which, through its habits, makes very little use of sight.

• It has only very small eyes.

The aspalax of Olivier (Voyage in Egypt and in Persia, II, pl. 28, f. 2) lives underground like the mole.

• It exposes itself to daylight even less than the mole.
• It has completely lost the use of sight

The Proteus is an aquatic reptile related by its affinities to the salamanders.

• They live in deep and dark cavities under the water, like the aspalax, has only vestiges of the organs of sight, vestiges which are covered and hidden in the same manner.

Light does not penetrate everywhere.

Consequently, animals who live in the dark do not exercise their eyes.

This makes the eyes shrink and even disappear.

Eyes disappear, reappear, and disappear again as a result of the possibility or the impossibility of the animal’s making use of it.

In the acephalid mollusks, the great development of the mantle has made their eyes and even their head entirely useless through a constant lack of use.