Theory:

Lamarckism includes four main principles as follows:
  1. Internal vital force
  2. Effect of environment and new needs
  3. Use and disuse of organs
  4. Inheritance of acquired characters
1.  Internal vital force:
  
According to Lamarck, every organism has a vital internal force. The size of living and their parts tend to increase in its volume throughout time. This growth in size is due to the organisms internal vital force.
 
2.  Effect of environment and new needs:
 
Our environment is a tremendous influencer. Hence, a change in the environment brings about initiation of new needs in the organisms. Inorder to meet up these new needs organisms develop certain adaptive characters which are morphological or anatomical. This leads to changes in the functions also. Such variations are called acquired characters. These adaptive characters of the organisms may be in the form of development of new parts of the body.
 
3.  Use and disuse theory:
  
According to this theory, an organ that is used constantly developed well and is strengthened. The organs which are not used for an extended period of time gradually degenerates.
An evidence of this theory was the evolution of elongated necks in giraffe and degenerated wing of Kiwi as they have adapted to walk.
 
Similar to the horses, the ancestors of giraffes wear bearing short necks and short forelimbs. But due to the lack of surface vegetation, they forced themselves to stretch their neck and forelimbs to feed on the leaves from the trees.
 
This continuous stretching of their neck and forelimbs resulted in the development of long neck and long forelimbs, considered characters.
 
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Image indicating the elongation of neck in giraffe
  
4.  Theory of inheritance of acquired characters:
  
As discussed above, acquired characters are developed in animals to respond to the change in the environment.
According to Lamarck, as these characters provide better chances of survival for that species, they are transmitted to their offspring by inheritance.

The evolution of long necks and forelimbs in giraffe was useful. Hence, they were inherited by their later generations.
 
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The principles of Lamarckism