Lamarck’s Theory: Evolution Through Acquired Traits
Lamarck’s Theory of Evolution
Lamarck formulated the first comprehensive theory of evolution. He proposed that the vast diversity of organisms, then believed to be static creations of God, had evolved from simpler forms. He posited that organisms themselves drove this evolution through their ability to adapt to their environment. Changes in the environment generated new needs, leading to modifications that were then inherited by subsequent generations.
His theory was supported by the existence of fossil remains of extinct intermediate forms. However, it faced opposition from the prevailing belief in the unchanging nature of species since creation, as well as from the influential Cuvier, who attributed species loss to geological disasters rather than evolutionary transitions.
Lamarck’s theory focused on the evolution of life, not its origin, which he believed arose spontaneously in its simplest forms. It wasn’t until 50 years later that Pasteur demonstrated that fermentation and organic decomposition were due to the action of living organisms, disproving spontaneous generation.
Lamarck developed his theory during a time when the natural sciences were still in their infancy. His observations of organisms perfectly adapted to their environments led him to consider two possibilities: either organisms were created with all the necessary adaptations for all existing environments, and these environments remained unchanged since creation, or organisms adapted to these environments, modifying their structure as the environment changed, as he proposed.
While Lamarck addressed the evolution of life, he also developed a mechanism by which it would evolve. He believed that nature acted through trial and error. His theory can be summarized as follows: circumstances create needs, these needs create habits, habits lead to changes through the use or disuse of particular organs, and nature is responsible for solidifying these changes.
He described this development as a result of six points:
- All organized bodies (organisms) on Earth have been produced by nature over a vast period of time.
- Nature constantly begins, and even restarts daily, to form simpler organized bodies, which are essentially initial drafts created through spontaneous generation.
- These initial sketches of animals and plants gradually develop organs and eventually diversify.
- The inherent power within each organism gives rise to different modes of propagation and regeneration, preserving acquired progress.
- Over time, circumstances and changes in the Earth lead to new situations, requiring organisms to maintain their diversity.
- Changes in the organization and its parts, known as species, are formed gradually and successively. Species possess only relative constancy and may not be as old as nature itself.
Lamarck’s Two Laws of Evolution
Lamarck formulated two key laws:
First Law: Use and Disuse
In any animal that has not reached the limit of its development, the frequent and sustained use of any organ gradually strengthens it, increasing its output in proportion to the duration of use. Conversely, the constant disuse of an organ weakens it and can even lead to its disappearance.
Second Law: Inheritance of Acquired Characteristics
Everything that nature has caused individuals to gain or lose under the influence of their environment, and consequently through the predominant use or disuse of an organ, is preserved in the next generation, provided that the acquired changes are common to both sexes or have produced the new individuals.