What was the initial, ancestral split among Spaniels based on, before size differentiation?

Answer

The environment where they executed their duties

Centuries ago, the earliest ancestors of the modern spaniels were functionally divided based on the terrain they were expected to work on. This led to the categorization of land spaniels, designed for terrestrial environments, and water spaniels, specialized for aquatic retrieving or work near water bodies. The modern Springer traces its ancestry directly back to the land spaniel designation, indicating a historical focus on working inland cover where flushing was the main requirement.

What was the initial, ancestral split among Spaniels based on, before size differentiation?
animalDogbreedlineagespaniel