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Lamarck was a French naturalist who published Zoological Philosophy in 1809. He was the first thinker to come up with a reasonable theory of organic evolution. He believed that species could change through time by passing on traits acquired during an individual's life to their offspring - the so-called "acquired characteristics".
In Lamarck's view organic beings constituted a ladder of life from simplest to complex animals, with humans at the top rung. In this way all variation now and in the past were united by a "Great Chain of Being". This was the Scala Naturae in motion. Lamarck did not really explain the origin of this ladder, nor did he acknowledge the possibility of a species becoming extinct. But he did offer an explanation for how organisms change over time, thus turning the ladder into what we might call an escalator of being.
The theory of Lamarckian inheritance, or the inheritance of acquired characteristics, remained influential and popular until the late nineteenth century. The belief that characteristics acquired by organisms in response to the conditions of life or as a result of their own habits could be inherited by their descendents, was shared by both Lamarck and Darwin. The theory fell out of favor with the rise of knowledge of hereditary and genetic science. One version, known as Lysenkoism, was popular in Stalinist Russia, it put Soviet agriculture back several decades relative to the West.
Interestingly modern advances in DNA sequencing and other aspects of molecular biology reveal that certain acquired structures of the immune system may be transferred from parent to child, defying commonly held evolutionary beliefs and recalling Lamarck assertion of the inheritance of acquired characteristics resulting from environmental factors. - see the book Lamarck's Signature, by Edward J. Steele, Robyn A. Lindley, and Robert V. Blanden (Perseus Books, 1998)
Jean-Baptiste Lamarck (1744-1829)
Lamarck and His Theory of Evolution
Chevalier de Lamarck
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