The first theory of organic evolution was postulated by a French biologists Jean Baptiste Lamarck in 1809 A.D., which is popularly known as Lamarckism. Lamarckism is also described under the name of “inheritance of acquired characters”.
FACTS OF LAMRCKISM:
Lamarckism is based upon following important facts:
a) Tendency to grow:
The internal forces of and organism cause to increase in the size of parts of an organism. The new structures appear because of an inner want of the organism. The internal forces of life tend to increase continuously the size of an organism and its component parts. This increase in size is up to a certain limit which is determined by the life itself.
b) Environmental Effects:
The environment in which the organisms liv influences them and this influence leads to change in their habits. Then, the organs of an animal became modified in appropriate fashion in direct response to a changing environment. This results the formation of new organs.
c) Use and disuse of organs:
According to the Lamarckism, the efficiency and development of an organ is directly proportional to its use and disuses. The more continuously used organs become more developed and enlarged while unused ones gradually weakens and finally disappears.
d) Inheritence of acquired characters:
According to this, organisms gain some characters during their life span from the surrounding and the changes in the surrounding. In a long course of time, organisms become entire different from their ancestors with new characters acquired from the pre-existing organisms.
Examples of Lamarckism:-
a) Formation of web toes in water birds:
The present water birds and ancestors were the terrestrial birds or land birds. Some of them could not find food in sufficient amount on the land and migrated into water in the search of food and security. They spread their toes while moving in water. The skin extended between the toes and formed webs for swimming. Hence, the present water birds are accompanied with the web toes.
This illustrates the law, “formation of new organs or environmental effects”.
b) Formation of long necked and legged giraffes:
The horse like short necked giraffes were the ancestors of the present long necked and legged giraffes. They used to graze on ground. But during certain course of time, there became the scarce of the grass on which they feed. Due to this, they had to depend on the foliage of the trees. As the trees were high and big, they continuously have to stretch their neck and legs while eating. Due to continuous use of necks and leg for stretching, their neck and leg gradually became long and strong. In a long course of evolution the present giraffe with long neck and legs are evolved.
This illustrates the law, “Use and disuse of organs” and “Inheritance of acquired characters”.
c) Loss of limbs of snakes:
The ancestors of the present limbless snakes with limbs. In the course of time, those snakes had to live in bushes and burrows. During which their body became elongated as they had to pass through them. For passing through the narrow spaces their limbs were not used continuously and finally, limbs were disappeared by continuous disuse. Thus, present snakes are limbless unlike other reptiles.
This illustrates the law, “Use and disuse of organs”.
Objection to Lamarckism:
The following are the objections to the theory of Lamarckism:-
- Our eyes are continuously used for watching & as told by the law; “use and disuse of organs”, it should be developed. But the eyes become more and more weaker as we grow up older.
- The organs in the organisms which are useless in the initial stages of life do not vestige but develop later on in the body and become functional.
- The characters acquired by the organisms from surrounding are not found to be heritable. Only those characters affecting germ cells are found to be heritable.
- Mutilations are not inherited from parents to offspring.
- Only the formation & development of organs due to environmental effects have been explained but not their roles in incipient stage.