Theory of organic evolution of species by Darwin and Wallace doesn't explain all of these except

Options

A. arrival of the fittest .
B. the development of vestigial organs
C. over - specialization of some organs like tusk of elephants
D. the cause of struggle for existence.

Answer

To solve the question regarding the theory of organic evolution by Darwin and Wallace, we need to analyze the options provided and understand what the theory explains and what it does not.

### Step-by-Step Solution:

1. **Understanding the Theory**:
- The theory of organic evolution proposed by Charles Darwin and Alfred Russel Wallace is primarily based on the concept of natural selection. It posits that species evolve over time from common ancestors through a process of modification and adaptation to their environments.

2. **Key Concepts**:
- **Descent with Modification**: This means that current species have evolved from pre-existing forms (ancestors), and they are modified versions of these ancestors.
- **Survival of the Fittest**: This phrase, often misattributed to Darwin, refers to the idea that organisms best adapted to their environment are more likely to survive and reproduce.

3. **Analyzing the Options**:
- **Arrival of the Fittest**: This term is not part of Darwin and Wallace's theory. Instead, the theory focuses on the survival and adaptation of existing organisms rather than the emergence of new ones.
- **Development of Vestigial Organs**: This concept relates to evolutionary remnants that have lost their original function, which is explained by evolution but not specifically by Darwin and Wallace.
- **Over-Specialization of Organs**: This refers to the excessive adaptation of certain traits, which is also not a focus of Darwin's and Wallace's original theory.

4. **Identifying the Correct Answer**:
- The question asks for the aspect that the theory does explain. Since "arrival of the fittest" is a misinterpretation and not part of the theory, it is the correct choice for what the theory does not explain. The other options relate to evolutionary concepts but are not explained by Darwin and Wallace's theory.

5. **Conclusion**:
- Therefore, the correct answer to the question is that the theory of organic evolution by Darwin and Wallace does not explain "arrival of the fittest."

### Final Answer:
The theory of organic evolution of species by Darwin and Wallace doesn't explain all of these except for the cause of struggle for existence.