Theory of organic 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 what the Theory of Organic Evolution by Darwin and Wallace explains, we need to analyze the options provided and determine which one aligns with their theory.

### Step-by-Step Solution:

1. **Understanding the Theory of Organic Evolution**:
- The Theory of Organic Evolution proposed by Charles Darwin and Alfred Russel Wallace states that modern species have evolved from pre-existing forms through a process of natural selection. This theory emphasizes the struggle for existence and the adaptation of organisms to their environment.

2. **Analyzing the Options**:
- **Option 1: The arrival of the fittest**: This concept is closely related to natural selection, which is indeed a part of Darwin's theory. It suggests that those individuals best adapted to their environment are more likely to survive and reproduce.
- **Option 2: The development of vestigial organs**: This concept refers to organs that have lost their original function through evolution. While this is an important aspect of evolutionary biology, it is not specifically addressed by Darwin and Wallace in their original theory.
- **Option 3: The over-specialization of some organs (like the tusk of an elephant)**: This idea is more aligned with later evolutionary theories and is not a primary focus of Darwin and Wallace’s work.
- **Option 4: The cause of struggle for existence**: Darwin and Wallace explicitly discussed the struggle for existence as a key component of their theory, highlighting how competition for resources leads to natural selection.

3. **Identifying the Correct Answer**:
- Among the options, the only one that is clearly explained by the Theory of Organic Evolution by Darwin and Wallace is **Option 4: The cause of struggle for existence**. The other options either relate to later theories or are not directly addressed by Darwin and Wallace.

### Conclusion:
The correct answer is **Option 4: The cause of struggle for existence**.