Evolution is considered to be a unifying principle of biology since it explains both the unity and diversity of life.

Answer

### Step-by-Step Solution:

1. **Definition of Evolution**:
- Evolution is defined as the gradual change in the characteristics of species over successive generations. This change can occur due to various processes, including natural selection.

2. **Continuous Process**:
- Evolution is not a one-time event; it is a continuous process. Changes in species occur gradually over time, adapting to their environments.

3. **Unity of Life**:
- Despite the changes that occur through evolution, there is a fundamental unity among all living organisms. This unity is reflected in the fact that all living beings share common ancestors.
- For example, all organisms have DNA (or RNA in some cases), indicating a shared genetic constitution. This commonality suggests that there is a basic similarity in the biological makeup of diverse species.

4. **Diversity of Life**:
- Evolution also explains the diversity of life. Different species adapt to their specific environments or habitats, leading to a wide variety of forms and functions.
- For instance, humans living in different geographical areas exhibit variations in traits that have evolved as adaptations to their surroundings. Similarly, animals and plants have evolved distinct characteristics based on their habitats.

5. **Conclusion**:
- Therefore, evolution serves as a unifying principle in biology, explaining both the unity (common ancestry and genetic similarities) and the diversity (adaptations to different environments) of life.