The fallacy in Lamarck's evolutionary theory was the assumption that

Step 1: Analyze the question The question asks for the fallacy (the mistaken idea) in Lamarck's theory of evolution.

Step 2: Explain the concept Lamarck proposed two main ideas:

1. Use and disuse: Body parts that are used often become stronger and bigger, while those not used weaken and shrink.
2. Inheritance of acquired characteristics: Changes an organism picks up during its lifetime are passed on to its offspring.

A classic example is the giraffe. Lamarck said giraffes stretched their necks to reach high leaves, and their offspring were then born with longer necks.

Step 3: Find the flaw Modern genetics shows that only changes in the genetic material (DNA in the gametes) can be inherited. Changes to body cells (somatic cells) during a lifetime, such as bigger muscles from exercise or a lost limb, are not passed on to offspring.

Now we check each option:

• A. Acquiring traits through disuse is part of Lamarck's idea, and it is not the central error. Organisms do change with use and disuse during their lifetime.
• B. Acquired traits are heritable. This is the actual fallacy, because acquired traits do not alter the genes in the gametes.
• C. Lamarck did not say acquired traits are seldom formed, so this is not his assumption.
• D. Acquiring traits through use of body parts is observable, for example muscles growing with exercise. The mistake lies in believing they are inherited.

Step 4: Answer

Tip: Remember Lamarck as "use and disuse + inheritance of acquired traits." The fallacy is always the inheritance part. Compare this with Darwin, who proposed natural selection acting on inherited variations, and with Weismann, whose experiment of cutting off mouse tails over many generations without any effect on offspring disproved Lamarck's idea.