Jean-Baptiste Lamarck (1744–1829) was a French naturalist who provided one of the first comprehensive theories for the transformation of biological species over time. His work, published in 1809, established a framework for biological change decades before Charles Darwin’s major publications on natural selection. Lamarck proposed a mechanism to explain how species could change in response to their environment, challenging the widely held belief that species were fixed. This early evolutionary theory suggested that changes acquired by an organism during its life could be inherited by its offspring.
The Two Laws of Lamarckism
Lamarck’s theory of evolution, often called Lamarckism, was built upon two primary concepts explaining how organisms adapt to their surroundings and pass on those adaptations. The first is the Law of Use and Disuse, which posits that an organism’s behavior directly impacts the development of its structures. Frequent use of a particular organ causes it to become stronger or more developed over the organism’s lifetime. Conversely, permanent disuse causes a structure to weaken, deteriorate, and eventually disappear.
This suggests that an animal’s needs, often brought about by changes in its environment, lead to new habits that either strengthen or diminish certain organs. The second, and most distinctive, component of the theory is the Law of Inheritance of Acquired Characteristics.
This second law proposes that the physical changes an organism acquires through use or disuse during its lifetime are automatically passed down to its offspring. For example, if an animal develops strong muscles through vigorous activity, its progeny would inherit this increased muscular development. The accumulated effects of these inherited modifications, occurring generation after generation, were thought to lead to the formation of new, well-adapted species.
Classic Examples of Acquired Traits
Lamarck used various examples to illustrate the application of his two laws, with the giraffe’s long neck being the most famous illustration. He suggested that the ancestors of modern giraffes had shorter necks, but were driven by a need to reach leaves on higher branches as ground-level vegetation became scarce. This continuous stretching and effort caused a gradual elongation of the neck and front legs in each generation.
The resulting longer neck, an acquired trait from the parent’s lifetime of stretching, was then passed down to their offspring. Over many generations, the accumulation of these small, inherited elongations eventually resulted in the distinctive long-necked giraffe seen today.
Another example involved aquatic birds, which were thought to have developed webbed feet because of the constant habit of stretching the skin between their toes while wading in water. Conversely, Lamarck pointed to the loss of sight in animals living in perpetual darkness, such as moles, as an example of disuse. In this case, the lack of necessity for vision caused the eyes to degenerate over time, and this reduction was then inherited by the next generation. Similarly, the long, slender bodies of snakes were explained by the continuous disuse of their limbs, leading to their eventual disappearance in their descendants.
Scientific Refutation of Lamarck’s Ideas
Lamarck’s theory was ultimately discredited because it lacked a verifiable mechanism for acquired traits to influence the hereditary material passed to the next generation. The rise of Mendelian genetics in the early 20th century established that traits are inherited through discrete, unchanging units, now known as genes. This understanding confirmed that an organism’s experiences during its lifetime do not alter the genetic information contained within its reproductive cells.
A significant empirical challenge to the inheritance of acquired traits came from the German biologist August Weismann in the late 19th century. Weismann performed a famous experiment where he cut off the tails of mice for multiple generations, yet the offspring were always born with perfectly normal tails. This result demonstrated that a major physical change to the body, a form of acquired trait, was not transmitted to the next generation.
Weismann’s work further proposed the concept of the “Weismann barrier,” which suggested a separation between the germline—the reproductive cells (sperm and eggs)—and the somatic cells that make up the rest of the body. This concept implied that changes to somatic cells could not affect the germline, thus preventing the inheritance of acquired characteristics. Darwin’s theory of Natural Selection, in contrast, explained adaptation through the selection of pre-existing variations within a population, rather than the acquisition of necessary traits by an individual during its lifetime.