Jean Baptiste de Lamarck (1744-1829) was a great French Zoologist and a pioneer evolutionist of the pre-Darwin period. In his book Philosophie Zoologique, published in 1809, he elaborated a new theory of organic evolution (evolution of plant and animals) based on the principle of use and inheritance of acquired characters. His theory was based on following main postulates:
- Effects of environment
- Use and disuse of organs
- Inheritance of acquired characters
Effects of Environment: Lamarck believed that evolution of new organs in many organisms takes place mainly because of some environmental changes. Some animals under some changed condition of environment require new physical development and acquire new characters, gradually, to better meet with the changed situation of food, habitat, physiology, or shelter.
Use and Disuse of Organs: Lamarck claimed that some organisms use some organs persistently while disuses others. The frequent use of any organ causes the evolution of that organ into a highly-build one. On the other hand, the disuse of any organ results in its degeneration and ultimate disappearance.
Inheritance of Acquired characters: Lamarck, in his theory, asserted that the accumulation of small changes, through successive generations, promotes the origin of new organs or characters which are transmitted to the offspring of the next generation. This is called inheritance of acquired characters.
Lamarck cited numerous examples to illustrate and support his theory of evolution; some are given below.
Evolution of Snake: In order to escape from the attacks of many mammals, the lizard-like and limed ancestors of snakes developed the habit of gliding over the ground and creeping through narrow fissures and holes. As a result, their body became elongated and their limbs disappeared due to their disuse, after a large number of generation.
Evolution of Giraffe: The ancestors of giraffe were horse-like and short-necked and lived in Africa having enough ground grass and herbage to eat. However, due to some drastic environmental change, they were compelled to graze upon foliage leaves of trees, thus, by the continues effort of stretching the neck and forelimbs resulted in the increase of the length of neck and forelimbs in each new generation. The necessity to get food in order to survive made the legs and forelimbs of giraffes enormously elongated.
Along with these examples, he also gave the examples of the evolution of kiwi, the evolution of the foot of a modern horse, the evolution of webbed feet of duct, the evolution of loss of teeth in the whale, etc.
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