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Write short note on Lamarckism


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Lamarckism, a theory of evolution based on the principle that physical changes in organisms during their lifetime—such as greater development of an organ or a part through increased use—could be transmitted to their offspring. The doctrine, proposed by the French naturalist Jean Baptiste Lamarck in 1809, influenced evolutionary thought through most of the 19th century. Lamarckism was discredited by most geneticists after the 1930s, but certain of its ideas continued to be held in the Soviet Union into the mid-20th century.

Biologists define an acquired characteristic  as one that has developed in the course of the life of an individual in the somatic or body cells, usually as a direct response to some external change in the environment or through the use or disuse of a part. The inheritance of such a characteristic means its reappearance in one or more individuals in the next or in succeeding generations. An example would be found in the supposed inheritance of a change brought about by the use and disuse of a special organ. The blacksmith’s arm (or any other set of muscles) enlarges when used continually against an external resistance, such as the weight of the hammer. If the effect were inherited, the smith’s children at birth would have unusually large arms—if not at birth, then when they became adults, even though they had not used their arms excessively. There is no evidence supporting this case. A more subtle illustration is found in the supposed inheritance of an increased dexterity of the hands of a musician through practice. The skill acquired, although causing no visible increase in the size of the fingers, might be imagined to be passed along to the musician’s children, and they might then be expected to play skillfully with minimal practice. Just how the intricate interplay of cerebral sequences that has given the dexterity to the musician’s fingers could ever be transferred to the musician’s sex cells (spermatozoa or ova), and through them to any potential children, has never been brought within the range of biological possibilities.

Lamarck recognized several ways in which the environment brings about changes in plants and animals, and it is significant to note that his attention was directed more particularly to the adaptive character of the response, which, as Henri Bergson points out, implies the teleological, or purposeful, nature of the result. In plants the response is direct and immediate; i.e., not through the mediation of a central nervous reaction system, since this is absent in plants. In animals the adaptive changes are supposed to be more indirect. According to Lamarck, new needs (besoins) arise in animals as a result of a change in the environment. This leads to new types of behaviour involving new uses of pre-existing organs. Their use leads to an increase in size or to other methods of functioning. Conversely, the disuse of other parts leads to their decline. It is the resulting material alterations that are inherited.

The examples that Lamarck gives to illustrate his doctrine are illuminating. In animals, as stated above, a new environment calls forth new needs, and the animal seeks to satisfy them by making some effort. Thus, new needs engender new habits, which modify the parts. The effects are inherited. For example, the giraffe, seeking to browse higher and higher on the leaves of trees on which it feeds, stretches its neck. As a result of this habit, continued for a long time in all the individuals of the species, the giraffe’s front limbs and neck have gradually grown longer. Birds that need to rest on the water—i.e., to find their food—spread out their feet when they wish to swim. The skin becomes accustomed to being stretched and forms the web between the toes. The horns of ruminants have resulted from the ruminants’ butting their heads together during combats. These examples, which appear naive in light of later discoveries, constitute some of the evidence on which Lamarck rested his theory.

Biology

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