Homologous structures are structures (i.e skeletal elements) that come from the same common ancestor but they appear in the descendants of the ancestor with some modifications, normally due to different functions.
The wing of bats, flippers of whales or dolphins and the forelimbs of horses is a common example of homologous structures. Bats, whales and horses are vertebrates and all came from the same vertebrate common ancestor.
The common ancestor of these organisms had the bones that make up these structures, but through evolution those bones that make up the homologous structure were modified to serve different functions. They were modified for the flight in bats, for swimming in whales and for life on land in horses.
Psychologists can usually tell if you have depression by asking you specific questions and doing a physical exam. Psychologists may, however, ask for lab tests to rule out other diagnoses. They will likely do blood tests to check for medical conditions that may cause depressive symptoms.
