The fitness, according to Darwin, refers ultimately and only to

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the meaning of "fitness" according to Charles Darwin. It is essential to clarify what Darwin meant when he used this term in the context of his theories.

2. **Context of Darwin's Work**: Charles Darwin was a naturalist known for his theory of natural selection. This theory explains how certain traits become more common in a population due to the advantages they confer in terms of survival and reproduction.

3. **Defining Natural Selection**: According to the theory of natural selection, organisms that possess advantageous traits are more likely to survive and reproduce. This leads to the idea that these traits will be passed on to the next generation.

4. **Clarifying "Fitness"**: When Darwin used the term "fitness," it is crucial to understand that he was not referring to social, mental, or physical fitness. Instead, he was focused on a specific type of fitness.

5. **Reproductive Fitness**: The term "fitness" in Darwin's context primarily refers to reproductive fitness. This means the ability of an organism to pass on its genes to the next generation. The more offspring an organism can produce, the higher its reproductive fitness.

6. **Conclusion**: Therefore, according to Darwin, fitness ultimately refers to reproductive fitness, which is the capacity to produce offspring that can survive and reproduce.

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