If we look at that region in mammals that don't see so well, such as bats, and we find that bats that don't see so well have the purple type, we know that this is probably what's causing this disease.
Hvis vi kigger på den region i pattedyr der ikke ser så godt, såsom flagermus, og vi finder ud af at flagermus der ikke ser så godt har en lilla type, vi ved at dette sandsynligvis er det der forårsager denne sygdom.