THEORY OF ORGANIC EVOLUTION -

LAMARCKISM

1. Theory of the inheritance of acquired characters or Lamarckism put forwarded by Jean Baptist de Lamarck (1809) in his book "Philosophic zoologique".
2. It was the first detailed theory about evolution.

CENTRAL THEME OF THIS THEORY -

1.       INTERNAL VITAL FORCE -

Organisms and their organs have a natural tendency to continuously increase in size generation after generation, generally making them large and more complex.

2.       DIRECT EFFECT OF ENVIRONMENT -

• The environment has a direct effect on the modification of the structures of an organism.
• Due to the changed environment, there is a new need for the organisms to adjust themselves accordingly.
• New needs result in changed habitat.
• It is known as doctrine of desires, i.e., appetency.

3.       USE AND DISUSE OF ORGANS OR PARTS OF THE BODY -

The structure or organ which is in very frequent use become more prominent and highly functional as compared to the organs or structures that are less used.

The new structures are the acquired characters and less used organs are reduced and ultimately disappeared.

4.       INHERITANCE OF ACQUIRED CHARACTERS -

The characters acquired by any species in its lifetime are inherited by the subsequent generations passed thereafter from generation to generations by hereditary factors.

Examples -

(i) Giraffe -

• Modern giraffes are considered as descendants of some short-necked animals.

• These ancestors used to graze on land and due to scarcity of grasses they started eating leaves of trees.
• This habit resulted in the use ofthe neck more and more and there is a slow increase in length of neck from one generation to next generation and ultimately giraffes with long neck appeared.

(ii) Amphibians -

• Ancestors of amphibians were aquatic fishes.
• Some of them started crawling on land and they need arms to crawl and lungs to respire in the changed habitat.
• This gave rise to fishes with lungs and lobed fins.
• There was no use of gills so they disappeared slowly.
• Later on limbs and proper lungs developed and amphibians came into existence.

(iii) Birds -

• Aquatic birds: They spread their toes widely and stretched the skin at their bases in order to rest on water.
• This gradually developed the webs between the toes.

(iv) Snakes -

• Snake: For the protection from enemies, they stretched their body to accumulate it in narrow burrows and did not use the limbs so finally body became longer and cylindrical while permanent disuse of the limbs caused their disappearance.