Lamarcks Theory of Evolution


 
 
Concept Explanation
 

Lamarcks Theory of Evolution

Lamarck proposed that the evolution of life forms had occurred by the use and disuse of organs. He gave the example of Giraffes, which initially did not have long necks. But, in order to reach the leaves on tall trees, they adapted by elongation of their necks. [By passing this acquired character to succeeding generations, over the years, they came to acquire long" necks. This theory, also called as Lamarckism is no more accepted now.

The four propositions of Lamarckism are as follows :- 

  • Living organisms and their parts tend to increase in size continuously due to internal forces of life.
  • New organs are formed in the body of organisms in response to a new need. For example, in the evolution of the horse, the plantigrade type of foot changed to unguligrade foot. The ancestor of the horse lived in soft ground for which the plantigrade foot was adequate. When the soft ground in the jungle was replaced by dry hard land, the foot changed to unguligrade type that suited running on hard ground.
  • If an organ is used continuously and constantly, it tends to be highly developed, whereas misuse results in degeneration.
  • Some examples are :- 

  • Aquatic birds develop webbed foot by stretching the skin between the toes.
  • Limbs are absent in snakes, Proteus and some burrowing animals since they were of no use in crawling and were a hindrance in movement.
  •  The long neck of the giraffe developed due to constant stretching to reach food (Fig. 8).
  • The muscles of the external ear or pinna in man are reduced but functional and well developed in animals like the dog, rabbit etc. to collect sound waves.
  • .... (More Text Available, Login?)
     
     
     
    Related Videos


    Students / Parents Reviews [20]