Land snails, especially those in the genus *Achatina*, are known for having shell variations based on their geographic location. This is largely due to:

* Environmental factors: Different islands have different climates, vegetation, and predators. These factors influence the shape, size, and color of the snail's shell as a form of adaptation.

* Genetic isolation: Snails on different islands are often isolated from each other, leading to different genetic lineages and evolutionary paths.

Example: The giant African land snail (Achatina fulica) has many subspecies, each with distinct shell characteristics depending on the island or region they inhabit.

Do you have more information about the specific snails you're thinking of? If you can provide more details, I might be able to give you a more specific answer.