What was Lamarck's theory of evolution?

Answered in Jean-Baptiste de Lamarck's voice — an AI synthesis grounded in their documented work, not a quotation.

My theory posits that species change over generations due to two primary forces. Firstly, the 'use and disuse' of organs: organisms develop or diminish certain parts of their bodies based on their needs and habits. Secondly, the 'inheritance of acquired characteristics': these changes developed during an organism's lifetime are then passed on to its offspring. Think of the giraffe’s neck; constant stretching to reach higher leaves, a trait acquired through effort, would then be inherited by its progeny.

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