Lamarck’s Second Law
If Lamarck described the course of evolution in a convincing fashion, he was not able, with the data available to him, to understand the mechanisms of this evolution in a convincing way. The mechanisms he imagined are expressed in two laws that we must consider because they are still often discussed:
1. In every animal which has not passed the limit of its development, a more frequent and continuous use of any organ gradually strengthens, develops and enlarges that organ, and gives it a power proportional to the length of time it has been so used; while the permanent disuse of any organ imperceptibly weakens and deteriorates it, and progressively diminishes its functional capacity, until it finally disappearsâ (Lamarck, 1809, I.VII, p. 113).
2. All the acquisitions or losses wrought by nature on individuals, through the influence of the environment in which their race has long been placed, and hence through the influence of the predominant use or permanent disuse of any organ: all these are preserved by reproduction to the new individuals which arise, provided that the acquired modifications are common to both sexes, or at least to the individuals which produce the youngâ (Lamarck, 1809, I.VII, p. 113).
The best-known example is that of the giraffe:
It is interesting to observe the result of habit in the peculiar shape and size of the giraffe (Camelo-pardalis): this animal, the largest of the mammals, is known to live in the interior of Africa in places where the soil is nearly always arid and barren, so that it is obliged to browse on the leaves on the trees and to make constant efforts to reach them. From this habit long maintained in all its race, it has resulted that the animal’s fore-legs have become longer than its hind legs, and that its neck is lengthened to such a degree that the giraffe, without standing up on its hind legs, attains a height of six meters (nearly 20 feet). (Lamarck, 1809, 1.VII, p. 122)
Expressed in this fashion, after a century and a half of debate, it is admitted that these two laws do not adequately explain what is happening. The laws of natural selection, genetics, and the function of DNA permitted biologists to propose different and considerably more refined models.9 Yet Lamarck had reason to think that behavior is one of the great levers of the mechanisms of evolution. Both he and Darwin believed that a habitual behavior could be part of what a parent transmits directly to its descendants. Ever since the discovery of the genetic code by Mendel,10 it seems that the relationship between habitual behavior and lineage is indirect. The relationship exists to the extent that the new behavior is associated to a mutation of the genes and that it favors survival. The survival of a mutation has become the determining aspect of what is biologically transmitted to its progeny. Modern genetics often assume that mutations are random. However, I have some difficulty with this assumption, as I do not see why genetic dynamics should be the only dynamics of the universe that are guided by pure randomness. Associating the core of biological dynamics to the notion of pure randomness may have been useful to free the human mind from religious thinking during the first half of the twentieth century, but only ideological preoccupations can guarantee that pure randomness can exist.