Jean-Baptiste Lamarck (1744–1829) was a French naturalist who proposed one of the first formal theories of evolution in his book Philosophie Zoologique (1809). Although later disproved, Lamarckism was historically significant as the first systematic attempt to explain how species change over time.
Lamarck proposed that body parts (organs) that are used frequently become stronger, larger, and more developed, while organs that are not used gradually weaken and eventually disappear over successive generations.
| Situation | Result |
|---|---|
| Organ used frequently | Becomes stronger and more developed |
| Organ not used (disuse) | Weakens and disappears over generations |
Example: Lamarck explained the absence of limbs in snakes by arguing that their ancestors stopped using their legs for locomotion. Over many generations, the unused limbs diminished and disappeared.
Lamarck proposed that modifications acquired by an organism during its lifetime — due to use, disuse, or environmental influence — are passed on to its offspring.
This means that if a parent develops a trait during its life, that trait will appear in the next generation.
Lamarck used the giraffe as his most famous illustration:
FBISE Exam Tip: The giraffe neck example is explicitly required by SLO B-11-B-06. Always explain it in terms of both laws: use (stretching) and inheritance of the acquired change.
Lamarck also proposed that organisms possess an innate tendency (sometimes called élan vital) to become more complex and better adapted over time. This internal drive, combined with environmental pressures, was thought to direct evolution toward greater perfection and complexity.
Lamarck's theory was eventually rejected for the following reasons:
Lamarck provided no explanation for how acquired characteristics could be transmitted from parent to offspring. Modern genetics shows that:
August Weismann provided direct experimental evidence against Lamarckism:
Weismann formulated the Germ Plasm Theory: only changes to germ cells can be passed to the next generation; the soma (body) and germ plasm are separate.
Lamarck had no knowledge of genes or DNA. We now know that:
| Aspect | Lamarck's Claim | Modern Understanding |
|---|---|---|
| Cause of change | Use/disuse + internal drive | Mutation + natural selection |
| Unit of inheritance | Acquired body changes | Genes in germ cells |
| Mechanism | Direct environmental effect on body | Random variation + selection |
| Experimental test | Not tested | Weismann's experiment disproved it |