Lamarck’s Theory of Evolution (Lamarckism)

Lamarck's theory rests on four main principles:

1. Growth: Living organisms and their internal parts have a natural tendency to continuously increase in size.

2. Formation of New Organs: A new organ develops in response to a continuous "new need" or want created by the environment.

3. Use and Disuse of Organs:

• Continuous use of an organ strengthens and develops it.
• Continuous disuse leads to its degeneration.

4. Inheritance of Acquired Characters: Any trait or modification an organism acquires during its lifetime is transmitted to its offspring through reproductive cells.

Lamarck cited several examples where environmental needs and "use/disuse" drove evolution:

• Giraffe’s Neck: Ancestors of giraffes stretched their necks to reach leaves on tall trees when ground vegetation became scarce. This continuous stretching was acquired and passed down, resulting in the long-necked modern giraffe.
• Evolution of the Horse: As soft jungles changed into dry, hard grassy plains, the ancestors of horses had to adapt. This led to longer legs, a reduction in the number of digits (shifting from flat-footed to running on hooves), and changes in teeth for grazing.
• Loss of Limbs in Snakes: Ancestors of snakes had short limbs. Continuous creeping through narrow holes and crevices made their limbs a hindrance, leading to their complete degeneration through continuous disuse.
• Other Examples: Webbed feet in swimming birds (developed by stretching skin between toes), reduced eyes in moles, and flatfish having both eyes on one side.

The theory was heavily criticised because it failed to explain how changes in body cells could alter genetics.

August Weismann's Rebuttal (Theory of Germplasm): Weismann dealt the greatest blow to Lamarckism by proving that acquired characters are not inherited.

The Rat Experiment: Weismann cut off the tails of rats for nearly 20 generations. If Lamarckism were true, the offspring should eventually be born tailless. However, rats were always born with normal tails.

The Scientific Conclusion: Modifications caused by the environment or the "use and disuse" of organs only affect somatic cells (body cells). Because these changes do not alter the germ cells (reproductive cells/gametes), they cannot be transmitted to the next generation.

• A blacksmith develops large biceps, but his children are not born with muscular arms.
• Boring of ears and nostrils in Indian women for centuries has not resulted in babies born with pierced ears.
• The ancient Chinese custom of women wearing iron shoes to keep feet small did not result in children being born with small feet.

• Organisms evolve by acquiring traits during their lifetime to adapt to environmental changes and passing these traits to their offspring.
• New organs develop in an organism as a direct result of a continuous "new need" created by their environment.
• Continuous use of an organ strengthens and develops it, while continuous disuse leads to its degeneration.
• Lamarck supported this with examples like giraffes developing long necks from stretching and snakes losing limbs from disuse.
• The theory was rejected after Weismann's rat-tail experiments proved that acquired changes only affect body cells and cannot be inherited.