State the best known contribution of
Hugo de Vries

Explanation

**Step-by-Step Text Solution:**

1. **Identify the Scientist**: The question asks for the best-known contribution of Hugo de Vries, a Dutch botanist and geneticist.

2. **Main Contribution**: Hugo de Vries is best known for proposing the **Mutation Theory of Evolution**. This theory suggests that mutations are the primary source of genetic variation in populations, which can lead to evolution.

3. **Key Features of the Mutation Theory**:
- **Mutations as Raw Material**: According to de Vries, mutations are the raw material for evolution, providing the necessary variations that can be acted upon by natural selection.
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