Explain the principles of Lamarckian theory.

Explanation

Principles of Lamarckism:
(i) Interl vital force: Living organisms or their component parts tend to increase in size continuously. This increase in size is due to the inherent ability of the organisms.
(ii) Environment and new needs: A change in the environment brings about changes in the need of the organisms. In response to the changing environment, the organisms develop certain adaptive characters. The adaptations of teh organisms may be in the form of development of new parts of the body.
(iii) Use and disuse theory: Lamarck.s use and disuse theory states that if an organ is used constantly, the organ develops well and gets strengthened. When an organ is not used for a long time, it gradually degenerates.
The ancestors of giraffe were provided with short neck and short forelimbs. Due to shortage of grass, they were forced to feed on leaves from trees. The continuous stretching of their neck and forelimbs resulted in the development of long neck and long forelimbs which is an example for constant use of an organ. The degenerated wing of Kiwi is an example for organ of disuse.
(iv) Theory of Inheritance of acquired characters: When there is a change in the environment, the animals respond to the change. They develop adaptive structures. The characters developed by the animals during their life time, in response to the environmental changes are called acquired characters. According to Lamarck, the acquired characters are transmitted to the offspring by the process of inheritance.