Evolution and Adaptation

by Thomas Hunt Morgan

2. That in her progress nature began, and begins still every day, to produce the simplest organisms, and that she still produces directly the same primitive kinds of organizations. This process has been called spontaneous generation.

3. That the first beginning of animals and of plants takes place in favorable localities and under favorable circumstances. An organic movement having once established their production, they have of necessity gradually developed their organs, and have become diversified in the course of time.

4. That the power of growth of each part of the body being inherited as a consequence of the first effect of life, different modes of multiplication and of regeneration have arisen, and these have been conserved.

5. That with the aid of sufficient time and of favorable circumstances the changes that have taken place on the surface of the globe have called forth new structures and new habits, and in consequence have modified the organs of the body, and made animals and plants such as we see them at the present day.

6. Finally, as a result of these changes that living bodies have been forced to undergo, species have been formed, but these species have only a relative constancy, and are not as ancient as is nature herself. If the environment remains the same, species also remain the same, as is exemplified by the animals living at present in Egypt, which are exactly like those living there in ancient times.

Lamarck concludes that the appearance of stability is always mistaken by the layman for the reality, because, in general, every one judges things relatively to himself. In fact, species are not absolutely constant, but are so only temporarily. “The influence of the environment is continuous and always active, but its effects may only be recognized after a long time.” The irregularity and the complexity of the organization of animals is the outcome of the infinitely diversified circumstances to which they have been subjected. These changes, Lamarck claims, do not directly cause modifications in the form of animals,[^[17]] but bring about changes in their needs, and changes in their needs bring about changes in their actions. If the needs remain the same, the acquired actions become habits. These habitual actions lead to the use of certain parts in preference to others, and this in turn to an alteration in form and structure. The individuals so changed breed together and leave descendants that inherit the acquired modification.

[17]. This is clearly meant to be applied only in the case of higher animals.

Curiously enough, Lamarck follows up this argument by citing some cases amongst plants that have been changed directly by the action of the environment. He says that since plants have no motions they have consequently no habits, but they are developed by changes in their nutrition, etc., and this brings about the superiority of some of the vital movements over others.

Amongst domestic animals Lamarck cites the case of the dog, that has come from a wild form like the wolf, but having been carried into different countries has acquired different and new habits, and this has led to the formation of new races, such as the bulldog, greyhound, pug-dog, spaniel, etc.

Lamarck’s argument shifts so often back and forth from animals to plants, that it is clear that in his own mind he did not see any important difference between the action of the environment on plants, and the use of the organs of the animal. He gives in this same connection his oft-quoted summary of what he calls the two laws of nature “which observation always establishes.”