A sea-horse can survive only in water and a horse can survive only in land why​

Seahorses have unique adaptations that help them survive in the ocean, including the ability to use camouflage, or blend in with their surroundings, and change the color of their body. Long snouts help them find food, and excellent vision and eyes that can move independently are great for avoiding predators.

In theory, wild horses survive by roaming large areas of land and grazing on whatever plant life is readily available and adequately nutritious. Horses are herbivores that can survive on a steady supply of healthy grass, shrubs and general plant life.

This is how sea horses survive in water and horses survive on the land

doctrine that the us had the right to expand throughout north america.