Final answer:
In biology, the characteristic not illustrating the principle of structure and function is a horse's mane being often black, as it does not relate a structure to a specific function.
Step-by-step explanation:
In the context of biology, structure and function are closely related. The question asks which scenario is NOT an example of this relationship. Option (a) A horse's mane being often black does not describe a functional aspect related to its structure, making it the correct answer.
In contrast, options (b) Wolves' eyes being forward-facing for focusing on prey, (c) Skin cells being flat and square to create a protective layer, and (d) Cows having flat teeth to grind forage are all examples where the structure of an organism or cell relates directly to its function.
For example, the forward-facing eyes of wolves (structure) are designed to help with focusing on their prey (function), skin cells are shaped specifically to form a tight protective barrier (function), and cows' teeth are flat to grind vegetation efficiently (function). These examples demonstrate the theme that the form of a structure in biology is typically linked to its particular functions.