Mutualism is an `ul(x)` whereas proto-co-operation is a `ul(y)` relation in which both interacting species are benefitted.

Answer

To answer the question regarding mutualism and proto-cooperation, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Define Mutualism**:
- Mutualism is a type of symbiotic relationship where both species involved benefit from the interaction. This relationship is often essential for the survival of one or both species.

2. **Provide an Example of Mutualism**:
- A classic example of mutualism is the relationship between figs and fig wasps. The fig provides a habitat and food for the wasp, while the wasp helps in the pollination of the fig.

3. **Define Proto-cooperation**:
- Proto-cooperation is a type of interaction that is similar to mutualism but is not obligatory for the survival of either species. Both species benefit, but the relationship is not essential for their existence.

4. **Provide an Example of Proto-cooperation**:
- An example of proto-cooperation could be the relationship between certain birds and large herbivores. The birds eat insects off the herbivores, benefiting from food, while the herbivores benefit from pest control.

5. **Summarize the Differences**:
- In summary, mutualism (X) is a mutually beneficial relationship that is often essential for survival, while proto-cooperation (Y) is a beneficial interaction that is not essential for the survival of the species involved.

### Final Answer:
- Mutualism is an **obligatory** relationship (X), whereas proto-cooperation is a **facultative** relationship (Y) in which both interacting species are benefitted.