Based on observation made during a sea voyage in a sail ship called H.M.S. Beagle around the world who concluded that existing living forms share similarities.

Answer

### Step-by-Step Text Solution:

1. **Understanding the Question**: The question asks who concluded that existing living forms share similarities based on observations made during a voyage on the HMS Beagle.

2. **Identifying the Key Figure**: The key figure associated with the HMS Beagle voyage is Charles Darwin. He was a naturalist who made significant observations during this journey.

3. **Observations Made**: During his voyage, Darwin observed various species of plants and animals in different environments. He noted that these living forms exhibited similarities and differences, which led him to think about their origins.

4. **Conclusion Drawn**: Darwin concluded that the living forms we see today have not always existed in their current form. Instead, they have evolved from earlier life forms over time. This process is known as evolution.

5. **Gradual Evolution**: His observations led him to propose the idea of gradual evolution, where life forms share similarities not only with each other but also with extinct species. This means that current species may have common ancestors with species that are no longer present.

6. **Disproving Special Creation**: Darwin's conclusions challenged the theory of special creation, which stated that all life forms were created as they appear today without any change or evolution. He provided evidence for the idea that life forms have changed over time through the process of evolution.

7. **Final Answer**: Therefore, the correct answer to the question is Charles Darwin.