Jean Baptiste Lamarck put forth the theory of inheritance of acquired characters, which is also known as Lamarckism. It was published in ‘Philosophie Zoologique’ in the year 1809.
The four propositions of Lamarckism are as follows:
a. Living organisms and their parts tend to increase in size continuously due to internal forces of life.
b. 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.
c. If an organ is used continuously and constantly, it tends to be highly developed, whereas misuse results in degeneration.
Some examples are:
i. Aquatic birds develop webbed foot by stretching the skin between the toes.
ii. Limbs are absent in snakes, Proteus and some burrowing animals since they were of no use in crawling and were a hindrance in movement.
iii. The long neck of the giraffe developed due to constant stretching to reach food (Fig. 8).
iv. 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.
d. Changes acquired during the lifetime of an individual are inherited by its offspring.
Criticism of Lamarckism:
Lamarck’s theory was subject to severe criticism. Two scientists Cuvier and Weismann were great critics of Lamarck.
Some objections raised against Lamarckism are as follows:
a. Though the tendency to increase in size has been shown in many forms, there are also instances where there is reduction in size. For example, trees that are primitive, are large in size, while the shrubs, herbs and grasses that evolved later are smaller in size.
b. If new organs were to develop in response to a new need, then man should have developed wings by now.
c. Changes acquired during the lifetime of an organism cannot be inherited by the offspring. For example, if a man loses his arm in war, he does not produce children without an arm. According to August Weismann, somatic changes acquired during the lifetime of the organisms are non- heritable, whereas, changes in the germplasm or reproductive cells are inheritable by the offspring.
Weisman theory is known as the theory of continuity of germplasm. August Weismann in 1904, removed the tail of mice for about 22 generations. The offspring of the 22nd generation also had a tail as long as in the original parents.
d. Mendel’s law of inheritance also disproved Lamarck’s theory.