This principle states that the structure of an organism or any of its parts is directly related to the function it performs. In other words, the shape, size, and arrangement of an organism's body parts are adapted to suit their specific roles.

Here are some examples:

* Birds: The hollow bones and streamlined body of birds are adapted for flight.

* Fish: The fins and scales of fish are adapted for movement and protection in water.

* Humans: The structure of the human hand with its opposable thumb is perfectly suited for grasping and manipulating objects.

This principle is fundamental to understanding how organisms have evolved to thrive in their specific environments.