THEORIES OF ORGANIC EVOLUTION


From time to time, various theories have been proposed to explain the mechanism of origin and evolution of new species. These include: LAMARCKISM Environment effect J.B. de Lamarck (1744-1829), a French naturalist, who introduced the word biology. He proposed the theory of 'Inheritance of Acquired Characters' in his book entitled Philosophic Zoologique. He was of the view that: Effect of use and disuse organ: Environment influences change of characters of and organism due to change in their needs e.g., in dry soil, the root system of a plant becomes more extensive than in wet soil; plants grown in shade develop larger leaves than those grown in open etc. Constantly used organs become highly developed and modified in response to needs,while those disused result in to degeneration. Lamarck cited the example of evolution of Giraffe, presumably from the ancestors having equal sized fore and hind limbs and a short neck. The Giraffe lived in African deserts, which have no vegetation except a few trees. Their continued efforts through many generation for getting food from high up foliage, resulted in to longer fore legs and much elongated neck. Similarly, the absence of limbs in snakes and presence of vestigial organs in animals have been explained by Lamarck on the basis of their disuse. Finally, the modifications which an organism acquires during its life time, gets inherited by its offsprings.

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