Lamarck's Position
Jean-Baptiste Lamarck's evolutionary theory, set out in Philosophie zoologique (1809), made four substantive claims.
Inherent tendency toward complexity. Lamarck held that life has an inherent directional tendency toward greater complexity over time. This claim is rooted in eighteenth-century natural philosophy and reflects assumptions about progress in nature that do not survive in modern biology in the form he proposed.
Use and disuse. Lamarck held that organisms respond to environmental conditions by developing new needs, which drive the use or disuse of organs. Frequently used organs strengthen and elaborate; disused organs weaken and disappear. This is a claim about within-lifetime developmental response to environmental conditions.
Inheritance of acquired characteristics. Lamarck held that traits acquired during an organism's lifetime through use and disuse are transmitted to offspring. This is the "soft inheritance" thesis, often summarized in the (incorrect) cartoon example of giraffes passing stretched necks to their descendants.
Directional adaptive evolution. Lamarck held that the combination of use-and-disuse with soft inheritance produces evolutionary change that is specifically tuned to environmental conditions. Evolution, on his account, is directional and adaptive in a strong sense.
These four claims together form the position called Lamarckism in later usage. The neo-Darwinian synthesis of the early twentieth century rejected this position, primarily on the grounds that the Weismann barrier prevents somatic adaptations from reaching the germline. Random mutation followed by selection on phenotype became the canonical evolutionary mechanism.
This rejection was substantially correct as a claim about specific mechanism but too broad as a claim about the underlying phenomena Lamarck was attempting to describe. Several twentieth and twenty-first century developments have restored elements of what Lamarckism was reaching for, without restoring its specific mechanism.
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